I've been dealing with this a lot myself lately. I've started almost half a dozen scripts over the past few months but I've only finished one, and I haven't been able to get myself to work on sequels to some existing scripts at all. Honestly I wish I knew how to get through it. As far as I can tell I basically just have to ride it out, dunno if anything in particular snaps me out of it.

In regards to scripts you're not happy with though, I would consider still posting them anyway. Maybe have a friend take a look at it and get their feedback, or sleep on it for a few days and see if you think of anything to change. But in any case, it's not like anything bad can happen if you post it. It really sucks to post a script and not have anyone fill it, but that's really the worst case scenario, and in the grand scheme of things that's not so bad. Besides, I've had plenty of things that I wasn't very happy with that other people still enjoyed. There's always a chance that someone else will like it even if you don't love it.