The best thing I can tell you to do is stop and take a break if you get frustrated. Park somewhere out of the weather so you aren't fighting rain, and take your time. It will help a lot if you do it somewhere warm so the seal is more pliable, and you may want someone to help you. Once you get it done you will realize that it was pretty easy, but it takes a bit of time to get used to doing it and figure out the trick, and you may have to stop when you are halfway done and start over.
I'm not overly patient either, but I eventually got it. I know the manual says to put the adhesive on before you put the gasket in, but I found that that doesn't work too well, since the seal shifts most of it around. Forcing it in after you put it in with a caulk gun works a lot better.
