Thanks for contacting Just Answer. I think I can help you here....
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OK... I have read you post a couple of times. I see you are in quite a tough situation to say the least....
Several things come to mind....
One should you tell him? Well...... If you want to make a life with him and if there is no chance that he will find out from someone else, you are right to keep this to yourself - as you say, you don't want to put him through the awful emotional issues that he put you through previously.
If you want to stay with him and there IS a chance that he will find out - then of course it is best that he finds out from you. This cannot be overstated!
If you do want to stay with him... you will need to forgive yourself for what you did and make a real and very conscious decision to be faithful from this point on.
Now, these feelings you are having for your friend and quite understandable.... but not necessarily love or even good...
Of course it was fun! It was SEX - Sex is fun. And what you did was "forbidden, impromptu, clandestine" and so on... It's exciting to think that someone other than our significant other wants us and is willing to risk something like this to show it.
Caution.... cheating can be like a drug - addictive :(
I work with people all the time who get caught up in the thrill - it starts out like what you have described here and then it escalates to various liaisons - just for the thrill... but it turns out that (like a drug) this starts to own them.
It won't be easy to walk away from this, but if you decide to do so, you should make a stronger commitment to your boyfriend.
You are certainly uncertain about your feelings as you have posted - this is quite common considering what has happened. I strongly suggest that you go see a counselor face to face to sort out your feelings - until you do that, you will have a hard time figuring out what you want.
I'm sorry that you find yourself in this very trying situation - but I really think a counselor can help. You are not the first one to be in a situation like this - there is hope.
I hope I have answered your question and you are satisfied with my efforts.
If you are please with my answer, please don't forget to click on the green accept button so I can get credit for my efforts.