So be single, but live your life, and find things you enjoy.... it seems you yourself let your friendships go due to possibly low self esteem but also to a serious lack of effort. I am married and middle aged and I have to reach out to my friends, stay in contact with them, make time for them and make an honest effort to maintain the friendships. Everyone is busy and/or married with kids, and it takes earnest and honest effort. It seems you didn't do that at all because you thought they didn't like you or were too busy for you and you gave all your time and energy to this man and your child. That's what you wrote in your first post.

"I invested so much into him, all I had, except for my child of course, I lost all other friends and I am basically alone. Everything revolves around him."

That was your mistake, You invested far too much into this person and you let the rest of your life fall by the wayside.

I understand how depression and loneliness work because I've been there plenty times myself, but you have to find the fight in you to get a life again for yourself. The only way to make a life for yourself is to take action, not be passive and just do it. At some point, you have to pick yourself up out of your slump and make an effort to work on your life.