I want to break up with him

This guy and I have been dating for 4 months. Idk something just seems off about the whole relationship, and on top of that I’m really unhappy. I feel bad though because he keeps talking about being with me forever and idk. I just don’t feel like he’s helping me become a better me and he makes me feel bad all the time. I don’t want to go into all that but I don’t know how to break up with him gently or what to say.

I struggle with a couple mental illnesses so this is really difficult for me. I don’t know what to do.

Can someone help me figure out what to say?

Or What do you do when you break up with someone?