What to do in this situation?(20F)Stuck in a cycle pls help out

20(F) I broke up with a guy I was seeing around 2 years ago. He was really in love with me, and ik how much it hurt him, but it just didn't work out. I simply didn't feel what he felt for me. Now he's back again after 2 years, and he's hella determined to get me back. Like really, really determined, he won't give up. Let's call him A.

In the meantime, I fell in love with someone else really bad and he broke up with me very recently. Let's call him B.

So now ik how much it hurts to be dumped by someone you really love and I hate doing this to A but at the same time I don't think it's a good idea for A being together with me again.

I want to be kind to A, can anyone pls tell me what to do without hurting him?