I'm having a terrible time learning to track stand properly.

Some background information. I had a major stroke in 2010, and my balance has never been quite right since then. I don't know how much that is affecting this. Basically, I can keep the bike upright, even through an entire light cycle, but I wind up creeping forward a couple of inches. I can't trackstand with a net + gain of zero. I tried again today, messing around in a parking lot--55 seconds, then I wound up flat on my back. So, I either move foward a tad, and stay up, or stay still and eventually fall. I've read a ton of other threads and tutorials and from what I can gather, I think the problem is that I can't really get the bike to roll backwards. How the heck do you do that? Oh yeah, also, I cannot do this standing at all. If I stand up to do it, I fall on to the top tube immediately every time. Seated I do much better, even though consensus seems to say this is harder. Thoughts, suggestions, tips? I'm totally getting frustrated...