I’m so sorry that you have been through such an ordeal. You were wise to go for some counseling right away. It sounds like you are well on your way to putting the assualt behind you and getting on with your life. Yours is a healthy, mature response.
Your boyfriend’s reaction, though embarrassing for him and frustrating for both of you isn’t that unusual. He blames himself for not being there to protect you and may not feel deserving of intimacy. He may be afraid he will remind you of what happened and hurt you. He might find it difficult to be sexual with you, knowing that someone else was – even though there was certainly no intimacy or love involved in the assault. Whatever the reason, he hasn’t been able to come to terms with it and you baven’t been able to help him. Ignoring the issue hasn’t worked either.
Since the relationship means a lot to both of you, I encourage you to get some additional counseling, this time as a couple. As you have both discovered, an assault may be on one person but it also affects the people they love. A therapist will help the two of you learn new ways to provide each other with support and care so you can get through this together.
I wish you well.