so do you want to continue the affair? let go of your bf and find a fling. obviously your bf is not the right one for you at this point in your life. but i understand that its difficult to let it go since you've invested your time and emotion on the relationship and its just the sexual aspect your'e missing. next option would be: you can keep the affair a secret and work out the sexual aspect with your bf and see how it goes...

another option would be to take a risk, let go both of them and invest on a new relationship where you have all the aspects of a relationship fullfilled. it may be a hard work, but on a long term it would be good for you.

P.S. (its ok if you want to let go the engaged guy after you have sex with him, just remind yourself that he is cheating on her gf).