I'm 19 and I'm lonely because I don't have any friends.
Why don't you have any friends?
I've been very mean to people and it's hard to make friends.
I'm sorry that you feel lonely but it's difficult to keep relationships with friends when you are mean to them and treat them poorly. Reflect on your behavior and see how you can change it so that you can maintain friendships. If you make friends, you'll be happy and won't have any reason to be mean to them.
I'll keep that in mind.
If you treat people better, you will be able to make friends and not be lonely.