How Do I Tell Guys About My Sexual Past?

Long story short, when I was 15 I lost my virginity, and shortly thereafter I went through an extremely difficult time in my life where I was severely overmedicated by a shrink, and experimented (regrettably) with cocaine and other drugs, not to mention drinking heavily. I ended up sleeping with more guys than I think anyone should at that age. I went through a period of recovery, and I saw how much that may hurt me, and saw how much being in steady, serious relationship to have sex was important to me. I have slept with 12 guys, and whenever I think about that, it makes me sick. Only 3 of the guys have ever been emotionally significant to me, the others were big mistakes caused by my life being out of control. I would give anything to take these guys back. But what do I tell future boyfriends when they ask how many guys I’ve been with? My last boyfriend was very surprised when I told him, and not in a good way, even after I explained. How should I handle this?
Just FYI, I’m 19 now, and the number is still 12, and I have never had and STD. And for those who think I still had control, I didn’t, I was on so many meds as a 100 lb 15 year old, that my docs told me that Vietnam Veterans don’t even recieve the meds and dosages I was on. I have no memory of most of that year, the meds made me have seizures etc., so that in addition to my own drug use was devastating. I really had no control at all.