How to crawl out of a deep dark hole of loneliness
In no particular order - and you might have to re-do some steps over and over until they take. This is written in a way I would have put it if I would address my past self. If you take umbrage with the tone, know it's not necessarily you I am talking to.
Take what applies, leave the rest.
Understand the difference between being alone and being lonely
Being alone is being by yourself. Loneliness is a feeling of lacking social connections or lacking in social connections. You can be alone and peaceful and content, you can be lonely amongst people.
Grieve the abandonment you have received.
No-one deserves to be lonely. Not you, not anyone. Whatever the reason for your loneliness may be, that remains true. Process your feelings, have compassion with yourself.
Realize that social connection is something that you want from other people.
Realize that other people don't owe you connection.
No-one deserves loneliness, but no-one is owed connection either. It is a harsh truth, but one that must be understood.
Come to terms with the fact that since you want social connection, it will probably be on you to reach out and put yourself out there until relationships have been established (and maybe even then - some people are wonderful, but they aren't good at reaching out and never will be).
Give yourself space to feel afraid. Have compassion with yourself. Reaching out is hard. Putting yourself out there is hard.
Keep in mind that your threat response might be higher than the average person's.
If you have been lonely for a long time, chances are your social skills are dusty and your threat response is high as a result (or maybe due to past trauma or maybe that's just how you're wired).
Chances are good you are perceiving other people's reaction towards you more negatively than they intend it to be. This will slowly, slooowwwwwly vanish with practice. And I'm sorry that this is how it is right now. I recommend looking into getting a CBT app - e.g. MindShift. It's free and it really helped me. (Note: the app is for dealing with anxiety and focuses specifically on helping you to re-evaluate your thinking patterns)
It helps to approach people with curiosity and love in your heart
Misanthropy is understandable. It's also not helpful in the slightest. If all of humanity sucks, what use would it be to try and strike up a conversation with any one person? Why would I try to talk to them if I assume that they have nothing interesting to say?
You have had love in the past, there is no reason why there can't be love again. There is something good out there and it is on you to find it.