He won't let go of his ex

My boyfriend of nearly 2 years insists on keeping in touch with his ex, something that has upset me from the beginning of our relationship. I feel threatened by her and don't trust her, as her need for his attention unsettles me. While she is in a new relationship, she still tries to keep ties with him and I suspect the second they have a fight she will be running to my boyfriend to cry on his should since he is "such a good friend." I have discussed this with him on numerous occasions which has led to huge fights, and I feel that by him sticking up for her and defending their friendship over my feelings he is choosing her over me. This really gets to me as I am not a possessive or demanding person, I am fine with all his other exes and women friends, except this one that I have a gut instinct about and have asked him to avoid just out of respect for me, but his refusal to do so has increased my feeling of powerlessness and suspicion. She is well aware of my attitude towards her, my boyfriend has told her about the fights we have had and I refuse to go to functions that she is attending just to try and avoid her.She has now invited us to her wedding. I have refused as I will not be a hypocrite and attend when I have made it clear that I wish she would just leave us alone. My boyfriend was still keen to go without me, until I said to him that I am very uncomfortable with that situation, and now he has said he will not go just to keep me happy. I don't want to have to be the person that forces my boyfriend to do as I want him to, but I am hurt that he seems to still put her first. What should I do?