My anxiety is messing up my relationship

The way I was brought up and my past relationships have given me really bad anxiety. I get scared to talk, and I get scared of confrontation, and I always think whatever I say will cause something. My relationship is suffering because of it, he wants me to talk more and communicate and hates that I'll stay quiet while he's talking about something. The thing is I want to talk but I overthink and scare myself into being silent or just responding with 'yeah' if I think saying something else will cause confrontation. I'm also scared of coming off as immature because I'm 18 and he's 29. Just this morning he was telling me some things he had issues with that I was doing and I got nervous and wasn't able to say anything back and I hated myself for it. I know that the only way for us to really communicate is for me to talk and express how I'm feeling and what I think about how he's feeling, and I know what I want to say, but whenever it comes time for me to actually do it I get nervous and just shut down. I feel so bad for it cause I know I'm the one that needs to make the change but I just don't know how to rewire my brain to know that confrontation doesn't necessarily mean he's going to leave or hit me.