I'm no good at this kind of thing, so I'll just come right out and say it. I don't think this relationship is working anymore and I think it would be best for both of us if we just stop seeing each other. I know this is not what you want, and believe me when I tell you that it is one of the hardest things I've ever tried to do, but you know as well as I do that it is time. Hell, we pretty much knew when we started seeing each other that we were never meant to be a long-term thing. Six months to a year - isn't that what EVERYBODY said? And we stretched it to over four years! Impressive.
I'm not going to try to sugar-coat this with BS like "it's not you - it's me," because truthfully, it pretty much IS you. It's you who made me all roid-ragey and emo. It's you who made me gain all that weight - presumably so no one else would look at me. It's you who made me cry at TV commercials, yell at EVERYBODY and everything, and feel guilty and crappy afterward. Did you HEAR some of the monstrous sputum I launched in the direction of my friends, coworkers and family alike? What was that?
Now, I already know what you're going to say, so I'll save you the trouble: You saved my life. Yes, you probably did, and for that I will forever be in your debt. But that does not give you the right to treat me the way you have - especially lately. The minute I hinted that I needed a little space, you went all Fatal Attraction on me. I'm not sure how many of your previous relationships that reaction has managed to save (I'm guessing none), but I promise you that it won't save this one.
Look - I don't want to fight about this. You're an amazing drug, and I have no doubt that you'll find another wonderful patient in no time. Let's just be mature, take a deep breath, and go our separate ways as friends, okay?