I cheated on my boyfriend

So basically I’m at uni and me and boy let’s call him boy B have been on and off since September and recently we made it official before this he had lots of commitment issues seeing and chatting to other girls but since we have been official he has been loyal. Me on the other hand I had slept with his friend let’s call him boy A on the first day of uni before I met boy B but boy A knows this and I had not spoke to him since till a few days after me and boy B made things official and long story short I slept with him. And because they both have the same friends boy B ended up finding out although he chose not to believe it and came and asked me and I denied it. Now I’m left feeling confused as he’s done me dirty in the past but technically we weren’t together and now I’ve done this I don’t want to tell him but I do have guilt and I fear he may find out the truth. I need advice.