If I know the knives will cut me, why do I stand there while they throw them?

Haven't posted in some time as life was hectic and I was doing well. Came to conclusion that relationship has no hope for the future - none. That is good, although at times depressing. Apparently I suck at long-term commitment, something upon which our culture places great value.
By accident (difficult to explain what that means) I met someone online for a casual/physically-based "thing." I have spent time with this guy - much of it in bed - great time spent in bed. Now I am wondering what the fuck am I doing. My previous relationship is not legally over yet, but over in all other ways. I am in major overload mode with work - major, major, major. And to add to it all I am bipolar.
Do bipolar people date? Is it possible for me to have any sort of "casual" thing with someone? I am a mess and always will be a mess. I have orange bottles of pills that remind me that every morning. I am having a good time with this guy on a number of different levels but am making it complicated in my head. I thought it was a wonderful idea at first - this guy thing. Now I am wondering if I am stupid as hell for jumping into something like this.
Why can't I just be happy with my dog? I understand him with ease.