relationship insecurity

I have been in a relationship with my boyfriend for over 4 months, my longest relationship yet. I never thought of myself as insecure, but he has girl friends and I have dreams at night about him cheating, and my thoughts about these girls who I have never met make me feel so insecure. I don’t want to feel this way, so I know I have to change my thoughts. I want my boyfriend to be able to have girl friends without me thinking this negative way and I don’t know how to change the way I am thinking to feel good about it. I want to feel better about myself and be confident in our relationship. I think about breaking up with him because my brain thinks it will be easier to deal with if I’m just not in a relationship at all, but I know that isn’t the right choice. Are there any specific ways that I can change my thoughts about myself, and him having these girl friends? Thank you! xx