Also, framing the problem a different way helped me put things in perspective.
"Given what I know now, if I wasn't already a part of this startup, would I join today?"
If the answer is "no", then you know what you have to do.
I was very recently in your position. My advice to you:
Get the f* out... NOW!
If I had to guess, you were probably never that impressed/excited by your co-founders to begin with. But you did it because the business-case made sense. That's what I did, anyways, and it was one of the biggest mistakes I've ever made.
If you think your co-founders are idiots, then it's over. If your experience is anything like mine, then not only are these guys not pulling their weight, but they probably have a lot of half-cocked ideas that you have to fight them on, they go off doing things that make no business sense, and have made you feel embarrassed for them (and your company) on more than one occasion.
In my case, I considered leaving my startup at the 15 month mark (and even before that), but hung around some more trying to make things work with co-founders who weren't doing their part. I had a very candid talk with them (not the first one either), laid out the things that needed to happen for me to keep going within what time frame, and everyone agreed.
Well, I did my part. They didn't. I knew, deep down, that they wouldn't hold up their end. If they were the type to do their part, they would've done it a long time ago. Denial is what kept me going for that last push, and it was a total waste of time and effort.
So I left. The co-founders balked, then got angry, then tried to get me to come back. I did not waiver. You shouldn't either.
The co-founders got over it. We worked out an agreement. We're still friends and things are fine. It will be fine for you too.
Now that I'm out, I'm watching their ship sink faster than ever. And I'm so glad that I'm not on it.
Hope my story helped you in some way. It was therapeutic for me. :) Now, get yourself out of there!