We are all shaped by our experiences (good, bad or otherwise) and if we were just able to forget of have memories erased, we would all lose the valuable lessons we spent that time learning. There is always something that you take away from a situation no matter how terrible or hopeless the situation looks.
There is one instance that pops into my mind when I think of things that I wish hadn't happened. A bunch of friends and I were in France and we were in this artist's village. We had just gotten lunch and then suddenly, we were surrounded by a bunch of guys. They said they wanted to make us wish bracelets, and that they were free, so we all said yes, they were free right…
Wrong! When they were done making the bracelets, they said now they would like a small donation. I would have been willing to give them a euro or two because the bracelet was interesting, but they kept pushing. Saying stuff like "that's all…" and there were tons of them, and I think between all of us, they got like fifty dollars because we wanted them to leave us alone.
It was a horribly frightening experience that I would love to forget, but you know what it did for me, I am really good at getting rid of telemarketers and walking right past people handing out fliers on the street and politely but firmly refusing the little piece of paper that I'm just going to throw out anyway.
So, as unpleasant as some experiences may seem at the time, you will realize that it is all for the best when you think about what you learned and not what you lost.