I think breaking up with my boyfriend was the biggest mistake I could've made.
Why did you break up with your boyfriend?
I didn't want to be with him because he wasn't honest with me.
It's understandable to feel like you've made a mistake after breaking up with someone. It's difficult to stop seeing someone you cared about, but the important thing to remember is that you have to do what's best for yourself.
I would marry him if I could and I can't stand the fact that I'm not with him.
Maybe you can talk to him and see if he wants to rekindle the relationship? I think it's fine you have regrets about the break-up, it happens all the time.