Why am I jealous of my boyfriend’s ex?

My boyfriend and I have been together just over a year now. We’re both 21. He was my first everything. Before me he had dated a girl for about 3.5 years before he joined the military and she cheated on him while he was away. They were each other’s firsts. For some reason whenever my boyfriend and I do anything all I think of is him being with her. From what he told me she was emotionally abusive towards him but at the time he loved her and he was planning on proposing to her eventually. It’s such a weird thing I’m feeling. It’s like even though I know they’re over, and they won’t get back together, I just feel like the replacement girlfriend. I’ll never be his first anything. Maybe that’s what bothers me so much. I don’t know. My boyfriend is amazing towards me. We love each other. We have talked about wanting a future together but for some reason I can’t get past thinking about him and his ex girlfriend. Any advice to help me get past the jealous feeling?