Hello, and welcome to me when I'm in the middle of a phase I'm pretty sure all animators go through when they work on one project for too long. The ever so consistant self question of, "does this suck?" its the dreadful point when all of the jokes you wrote wore off in their amusement and you start to question yourself.
You try to improve your self image of the cartoon, by showing the pencil test to your friends, but they simply lack the patience to watch something that isn't even close to done. This is where I am with Anime Amigos II right now. Its currently about five or six minutes in length and I'm half way done with inbetweening.
I've hit this with everything I've made, and for those other poor saps who are stuck in this ungodly rut, I offer some advice!
1. Don't show the unfinished project to other people, you'll be inevitably dissapointed by their reaction.
2. Re-record some lines, to add a fresh perspective to the cartoon. I just did this, its very effective.
3. Add subtle personal jokes when animating, ex: jiggle physics, hard nipples, thongs, and whatever other innapropriate shit that makes you laugh.
4. Watch any other material you like, for me this is usually stuff like Cowboy Bebop, Ren and Stimpy, or any other animated cartoons I like.
5. Start coloring some frames you know are finished or won't move. I find this to be a good and fun relief from the tedium that is inbetweens.
Although, as a foot note, you should also remember, that what your making might ACTUALLY suck. I've only had to abandon one half finished project because of this. If you really do look at your cartoon, disect everything, and decide it genuinely isn't funny...then you might be right. Thats the scary part of course. The idea of all that work being for nothing, but hey, it was a learning experience, so just move on to the thing you were gonna work on next. That next thing probably doesn't suck!
Heres a screen-cap of Anime Amigos 2 by the way! :)