From everything you're going through it comes down to one thing...if you want to stay in your relationship and have it thrive and flourish, you've got to learn to let what happened in the past go. Of course it's much easier said than done and I think we're all coming to grips with that.
In your case, your BF is back, but you let all the resentment, hate and anger that you held back when you were DBing fester inside of you so that it's not become this huge beast. Trust me, I know what you're going through. It is something that you can't get away from. It's definitely not as easy as saying "okay I forgive you, let's work it out". It take ALOT of patience and forgiveness on your part.
You will have certain "triggers" that will remind you of the things he did or something he shared with the OW. Kind of like post-traumatic stress disorder. And your BF will not understand because he didn't go through it from your POV. My W is the same way.
So it comes down to you. Only you can decide what you ultimately want. I think if you were to date other people, you were right in saying that it would be over. It's going to take alot of soul-searching on your part and how much you can forgive.
M-43 W-40 2D - 9 and 5
Emotion, yet peace. Ignorance, yet knowledge. Passion, yet serenity. Chaos, yet harmony. Death, yet a new life.