MDMA. I can't quite put my finger on it, but ever since I did my first ecstasy pill I was never quite the same. It's not like it changed my outlook on life or anything, I just felt like something toxic was happening in my head. I never really abused it to the extreme either, although I had little bouts where I'd do more than I should have. I have no idea why I kept on dropping pills, I guess I was in a haze at the time and didn't take much notice of what the shit was really doing to me. Though, I do admit, I had some good nights with it, I have very deep regrets about using it and I personally think it's an over-rated drug.
Marijuana. I have never come across any substance that is so psychologically addicting as cannabis (well for me anyways), and this is coming from a person that has tried a whole myriad of the 'harder' variety of substances, yes, including meth and Heroin. I have no idea why it had that psychological pull on me, maybe because I fooled myself into thinking it is a benign substance and an addiction to pot is relatively easy to manage. But, man, did that shit suck me in. Mind you, this is my own fault. No matter what activity I would do, I would have to smoke up before hand. I just kept on fooling myself into thinking, "mweh, it's only weed!", then before I knew it I was a paranoid mess of a person that couldn't get his shit sorted, couldn't even arrange a mere appointment and dreaded interacting with other physical forms of life.
I was self-medicating though, but was using the wrong substance.