Why can't you quit? You already gave your notice. If they are not trying to replace you, that's their problem, not yours. If you are short of money, can you get another job first, do some saving and pursue your idea on the side?
I've thought a lot about this since I posted this question. At the end of the day, I've given them notice till X. So, it doesn't sit right by me to not honor that :(
Yeah, it's naive, but I think getting out of this mess without devolving to their level means something to me.
This attitude is what got you into the mess in the first place. If you're not going to take the first step in trying to better yourself for your own good, then I have no faith in you.
If you're going to start a company, you're going to have to make hard decisions and have conviction. If you can't do even this when it's quite clear that it's negatively affecting you, then please don't start a company and just find another job.