There are several layers here, and I am very appreciative of your courage and resilience in discussing the issues. What is clear is that this isn’t a situation you alone got yourself into. Your boyfriend was very much part of the process and dynamics that got you into the dilemma in the first place. This wasn’t something you did on your own to benefit yourself.
There are three important questions here that you need to answer for yourself. First, is marriage what you want? Are you looking for that in your life at this time or is this relationship good enough for now. In other words, does the relationship you have with your boyfriend meet enough of your needs for now. If marriage is no longer a big deal for you then this might be okay enough.
Does your boyfriend’s unforgiveness knock him out of the running for a good long term partner? If you do want to get married does it feel as if your partner won’t be able to get over this? If marriage is what you want then let him know this in no uncertain terms. If he can’t forgive you and will hold resentment toward you then it is not worth going forward. He may feel that if he forgives you that he will lose his friends, or that he is settling rather than working through. But if he can’t get past this and you want to get married then you’ll have to let him know the relationship can’t go forward. In these terms, he can’t give you what you want and need.
Finally, I’d encourage you both to read this book Forgive For Good, and find a workshop specifically focused on forgiveness. In addition, you may want to find a couple’s counselor who can help the two of you work together on this.
There are two elements of forgiveness. There is forgiveness of the other and unforgiveness. The unforgiveness has little to do with the situation or another person. It has to do with the feelings you have that negatively ruminate about the situation. People can be unforgiving about someone who has passed on — or a situation that is long since dissolved but the bitterness remains.
Perhaps is a good place to start because your unforgiveness of yourself and your boyfriends lack of concern for you during that time may be beneath the whole situation. One way to break this cycle of unforgiveness is with self-compassion. As it’s essence self-compassion is being able to treat yourself like a good friend would treat you. Nurturing yourself in this way can ease the discomfort of the past — while setting the stage for not only being more tolerant of yourself and your boyfriend. Obviously, this works best when both of you work at this level.
Here is a brief video on self-compassion you may find interesting to help you connect with that part of yours or that needs nurturing.