Loneliness sucks. It's one of the worst feelings a human can go through.
And I know pretty well what I'm talking about. I'm alone most of the time. I fail at being a university student and never attend courses, I still live at home by my parents who work the whole day and because I'm an introvert with depression and social anxiety I also lost pretty much every friend I once had.
So being alone is good from time to time. You can focus on yourself, don't have to deal with others, relax.
But if being alone turns into a permanent condition, it gets bad.
It's actually scientifically proven that loneliness can increase your risk of dying. This study looked over 70 other studies covering over 3 million people.
[http://journals.sagepub.com/doi/abs/10.1177/1745691614568352?journalCode=ppsa]

They say that loneliness, social isolation and living alone are all associated with an early mortality, especially for people under the age of 65. All these factors have a higher risk to death than obesity and decreases your health like smoking 15 cigarettes a day.
Loneliness is also linked with an increased level of blood-clotting protein (Fibrinogen) which can cause heart attacks and strokes. As well it is associated with depression.
Humans are not meant to be alone, genetically we're pack animals like wolves or chimpanzees. If this gets taking away from us, we can't work properly anymore, our immune system has problems and our health decreases immensely.
Loneliness often affects more people around the age of 18-35 than elder ones and up to 80% of the adolescents reported to often feeling lonely. And for them it's hard to admit. When you're young, you're supposed to make a lot of friends. You're surrounded by people of your age at school, hang out with them and you should try out new hobbies where you can meet new people. But that often doesn't work out anymore and people will see you as a failure if you can't make friends as a teen. So we'll keep it for ourselves, won't get help, avoid people to not have to talk about what you did in your freetime and the lonely people will get even more lonely.
Loneliness becomes a public health problem worldwide.
So what can you do if you're lonely? I can't promise you that anything of the following list works properly for you. These are just things I could think of, including some of the things I do. Some of these are only short term measures, who might not help you in the long run

  • Movies/TV shows/Books: Anything that lets you escape your own world for a bit. This world can suck and doesn't matter how good your life is, everyone needs a break sometimes. There you can find companions who guide you and provide you with safety. So go fight some werewolves!
  • Music: It's treatment for your soul. It doesn't matter what kind of music you like, go and find that music that speaks to you. It's a place where you can be understood and you're never alone, because music connects people without the problems of language barriers.
  • Youtube: I spend a lot of time there till late in the night. Youtubers can become friends. In comparison to TV shows, they speak directly to you. It feels like having a friend with you. If you like a recommendation from me, I often watch gameplays, like 'Jacksepticeye', 'Markiplier' and 'Cryaotic'. Also Dan+Phil and if you're into Make-up go to 'Glam&Gore' They all offer a huge variety and it's always very funny and enjoyable
  • Online friends: A plus for them is they might be online when all irl friends are sleeping. For a lot of people it's easier to open up with people they don't know and can just write stuff online. Another plus is, it's just amazing to connect with people from all over the world and learn some new things. If you don't have some, there are tons of apps and websites, like HelloTalk or ConversationExchange. Also you can always send me a message, I'd love to talk with you. Since you know what kind of mess I am, it might be easier for you :)
  • Sports: I know it's hard to motivate yourself sometimes. But it's amazing for your health! And you feel so good if you use up all your power. If you don't want to go to the gym or join a sports club, home training works just fine. Jogging, yoga or bodyweight workouts are good to try out. If you more into dancing, here are two youtube channels with K-Pop workout/dances: https://www.youtube.com/channel/UC90qhppagynYF6wbqo6I9LQ, https://www.youtube.com/user/kpopxfitness
  • Hobbys: Get to know what you like and what topics you're interested in. Hobbys are likewise challenging and relaxing for yourself. Try to learn something new, it doesn't matter what. Learn how to play an instrument or a new language, start drawing or get to know everything about the solar system. Occupy yourself.
  • Caring for someone or something: Caring for others reduces loneliness. The best way it works is with family and friends. If that doesn't work for you, get a pet or - if you can't get one - visit an animal shelter. Often you can help out there and get to walk a dog. Also good places to care for someone are hospitals or retirement homes - just see if you're allowed to take care a bit. If nothing of that works, start with a house plant (maybe something more demanding than a cactus)
  • spend time without phone/tv and social media: Even though social media platforms are there to connect people and often it's a good thing to live with, sometimes it can increase loneliness. If you're feeling lonely, avoid pages like facebook where people brag about their great lifes. Knowing that there is a whole world right in front of you, can drag you down. So try to spend more time with real people than on the internet.
  • Open up: Smalltalk is a big factor of loneliness. It doesn't make you feel important, it's just casual talking without meaning. So go ahead and talk about more important things than just about school/work. Talk about your feelings or dreams and passions. It will make you so much more interesting and will make you feel better.
  • Holiday/Birthdays: Those are times where most people feel lonely. Plan ahead. Talk with your family or friends what you'll do at that time. It doesn't need to be something big, just to know that you won't be alone during these days helps.
  • Go out of your comfort zone: That's a really difficult thing to do. It's not easy to do things you don't specifically like, but most of the time it's worth it. Try new stuff, get to know more people. Don't be too afraid. It's okay to fuck up, just don't isolate yourself from the world. You're too precious to be hidden.
  • Call/message others: Talking as an introvert, this is ridiculously hard. But it's a big step out of loneliness. So go and message first. Even if the other person doesn't respond immidiately, that doesn't mean they're ignoring you or don't like you. They're probably just busy, so try again. It'll make them feel good and you get a chance to socialize. (But if they're still not responding, fuck them. You don't need these people in your life)
  • Quality/Type of relationships: These really matter. Always go for relationships with a higher quality than for quantity. If you socialize a lot but you can't connect with them, it'll still make you feel lonely. So sort people out who you can't really talk to and focus on people who matter to you.

If nothing of these helps, go get help. Therapy isn't a bad thing! It can be very helpful. Talking with a therapist can also be the first step out of loneliness.
Your health is the most important thing you have. Don't play with it and don't take it too easily. You matter and you're important. So stay alive as long as possible. I know that life isn't easy and lot of time it's very hard to deal with, but it still can be beautiful.
Please care for each other and care for yourself.
I love you ♥