How could I be confident?

By asking this question you have already started . Congratulations!.
I am sure others will jump in and offer suggestions for the rest of your journey

I would start off with any relationship slowly. Get to know this person and watch how they interact with people and see if they talk bad about anyone. If they do they are toxic. You need someone that will lift you up no matter what the situation is. To gain your self confidence back it takes some time. Hang out with people who care, make you laugh and feel carefree. Don’t worry what anybody else thinks of you. I am at the point in my life were I figure if they like me for who I am and what I have to offer then great, but if they do not, then they just lost out on a good and caring person.

You start out by trying to not think about the past. Don’t dwell on – or even think about – what happened before. I know it’s trite, but today is the first day of the rest of your life.

Then – incrementally – do things for you. Think a happy thought. Buy yourself a book. Flatter yourself. Take a walk along the river in a calming place. Yes, it’s sort of brainwashing yourself in a positive way, but the goal is to treat yourself well, and feel good about you, not anyone else.

Start small, do things incrementally. But you’re number 1 – not the past and the people from there.

I would slightly disagree with @spring25 – before you meet with positive people, you need to be happy with yourself in your own skin. So get your mind in order first, and then reach out to others.

But they key, at least to me, is – the past is over. It is not worth another thought. Move past.

Fake it till you make it.

Don’t fake it @Apparently_Im_The_Grumpy_One. Actually try to get out there and do something. Don’t care if you embarrass yourself or people laugh at you. Trust me, I’m the kid that’s bullied because I’m outgoing and I laugh at myself. Just keep trying to find ways to have fun!

My wife (then girlfriend) made me a parchment wall hanging with this on it. I still have it hanging in my library and look at it when I need an attitude adjustment.

Fear is the mind-killer.
Fear is the little-death that brings total obliteration.
I will face my fear.
I will permit it to pass over me and through me.
And when it has gone past I will turn the inner eye to see its path.
Where the fear has gone there will be nothing.
Only I will remain.

From the Frank Herbert book Dune

^ mad props for the Litany Against Fear.