I’m too paranoid to have a boyfriend

Hi guys, so I currently have a boyfriend & honestly I’m so paranoid with everything...I always think I’m going to get cheated on, I always think he’s going to find someone better than me. I never can sustain previous relationships because of my insecurities and fears of being left for something better I always end up sabotaging the relationship or feel the need to cheat first so I won’t feel the pain if they ever cheated on me. I know I need therapy. I currently have a boyfriend & I accuse him of everything. And he understands my trauma with not trusting men. He hasn’t left and wants me to have faith in him and not compare him to my past but it’s hard I’m trying not to give into my fears and try to hurt him first because he’s a good guy . How do you trust? How do you begin to? I need therapy & he says he’ll help me with that.