Yeah, I probably have been sad like that (but not clinically diagnosed), my childhood was a complete disaster.
But anyways, I don't want to lose the best years on this body on that nonsense, so I try to look forward to the future with an optimistic (but realistic) view. Do things you love with the good people that are around you, if you don't have anyone, try to meet someone, what else can you do?
It depends on how you are as a person, I am lonely by nature, so I'd rather have a great friend or someone who I really connect with, rather than 10 "friends" while not being that connected (at that point they're acquaintances).
When I'm alone I try to have fun with hobbies I like, or learning interesting stuff (although I have been slaking on that, lately), and relaxing when I can, give yourself some "me" time. A healthy diet, sleep and exercise also play a role, so you should try to not neglect those.
That should help you to improve your perspective in life, which is also a big factor (I now give less of a heck on certain things, because they're not worth the attention), if I focus too much on miserable stuff I'm wasting my time, I've already done that enough.