Please give some advice...how to get over past mistakes and bad choices?

It's embarassing that I have resorted to asking strangers about how to get over this stuff, but i am not comfortable talking about it with even my closest friends. so about this time last year i was having problems with my boyfriend and started experimenting with drugs, never got heavily into it or did it weekend but still drugs are bad haha. i ended up frequenting a bar where i was introduced to a dealer who of course, gave me free drugs cause im a girl. anyway long story short, i messed around with him once when i was still dating my boyfriend, and felt AWFUL about it. then we broke up and i had sex with this guy a couple times, and somehow i even let him take pictures of me and a video of us having sex. for some reason i trusted him, i dont know why. i have never seen these pictures and who knows, maybe they are on the internet but whatever. i normally would NEVER do this kind of stuff, i have no idea how he got me to do these things, might have just been the drugs or him making me feel good. i have a mutual friend who confided in me that she did the same thing with this guy, and has told me she feels the same way. he is just a manipulator and i never saw it. the thing is, i am back with my boyfriend, and have never told him about what happened. i wouldn't say it bothers me he doesn't know, even though we have talked about the guy and has said he doesn't like him and the fact that he takes advantage of girls he gives drugs too (me!). however, i think about the fact almost everyday i had sex with this person, and allowed him to take those pictures of me and such. i'm not that kind of person at all! for some reason i think about it daily and i feel sick, or when it pops in my head i'm just like "uggh i cannot believe i really did that". only about 3 people know about it all, and i would NEVER do anything like it again. any ideas on how to get over it or stop feeling so guilty?Please give some advice...how to get over past mistakes and bad choices?
You just have to tell yourself that it was a huge mistake and you are mature enough to admit that it was and you will never do it again. The past is the past. And mistakes is what makes us learn. I know your not gunna like this suggestion but maybe telling your boyfriend about it will make you feel less guilty at least.