I'm so ashamed to admit this- even to strangers in cyber space.
My boyfriend and I were together for 18 months. We did not have contact for about 8 months after breaking up and in that time I slept with somebody that he knows. I wouldn't say it was his FRIEND as such, but someone that he hangs with every now and again and someone he would speak to if they were both at the same party. Hooking up with that guy (my exboyfriends "friend") meant NOTHING and is by far the biggest regret of my life thus far. I can't bear thinking about it as the guilt just turns my stomach.
After about 8 months of no contact, me and my exboyfriend got back in contact. After awhile of talking we slept together a couple of times. If I'm honest, I never lost feelings for him but we both understand that we'd never get back together as we've accepted that we wouldn't be able to make it work.
I've considered telling him but worry that I'm only telling him to make myself feel better. I deserve the guilt and feel I should just live with it. However, I would not want this to come up somehow or for him to find out from other people when I really should have just done it myself! I know (despite this guy only being a close acquaintaince) my exboyfriend would be livid if he knew and by telling him I risk losing him from my life altogether. I realise this is purely selfish and I should probably allow him to make that choice and I should lie in the bed I've made for myself...
Should probably mention- the "hook up" happened over a year ago.
Any advice would be helpful, or if anyone has experienced a similar situation...

Why should you feel guilty since not only is it no ones business, but you were single, and free to do whatever you want. The real shame is trying to keep your ex in your life instead of moving on. That's what puts a glitch in the plan, you still have feelings, but you both know there is no relationship, but worse being intimate, knowing there will be no relationship. What's that?

Even you have to see that having just sex with an ex keeps the feelings stirred up, and keeps you from moving on to better things. Having sex is a lousy excuse to keep someone in your life, and for what purpose? All you get from this is a distraction from proper healing, and finding your true happiness some other way. That needs to stop, so you can put all that behind you and move forward to a healthier place with no guilt, or FEAR.

The others have already given you great advice. Just to view this from another angle, I would like to mention that you seem to be going in circles.

You're with your ex boyfriend, then you go with his friend, now back to your ex boyfriend. What's new? Another one of his friends? It's as vicious cycle. You already made it clear with your ex that it wouldn't work if you got back together.

I suggest you get out of this circle/cycle and meet new people. Make new friends and leave this past behind you as it doesn't appear to have any future.

Raven,
There is no reason what so ever for you to tell your EX-boyfriend whom you choose to associate with! You are no longer committed to one another in any way.
Do you really think he wasn't been with someone else during that 8month time frame. He doesn't feel the need to explain who,what & where to you. Reason being, you aren't a priority to him at this stage in his life, nor should you make him yours.
You both choosing to have once in awhile bootie call is not a relationship. You both can get this from a bar hook up.
Your all grown up and when you don't have commitment with someone special,you don't have to give a complete report.

I don't think that you should feel guilty about what you did considering that you two were not together at all (practically no contact) for a while and you were free to do whatever pleases you. But, if you feel like him finding out one day will jeopardize the relationship that you guys are having now whether it would be friends, friends with benefits, or to try an exclusive relationship again, then it will be better for him to hear it from you than for him to find out himself.

Good Luck,

Javi