Often, many of the most difficult decisions in life are not what you should do - because deep down, you already know - but how do it. This is true for your decision right now.
A few thoughts for you on that:
1. Irrevocably commit to doing it. Book a meeting with the founders to discuss "something important". Or tell your significant other that you've decided to exit by a certain date. Just do something that commits you to this course of action.
2. Get creative about why you're leaving. Instead of leaving because the company is a sinking ship, think of a diplomatic, but still truthful, reason for exiting. For example, focus on what you are going to do next, and supply that as the reason: "I'm much more interested in working on this kind of technology/product/etc. and I want to go pursue my dream."
3. If you can't bring yourself to tell them your decision face-to-face, put it in writing. Email them to tell them what you've decided, and close it with, "Can we talk?" Then hash it out with them. This is a great way to be done with something (see #1) because once the email is sent, it's sent.
4. Always remember that it is never as bad as you think it will be. It's just business, they will understand your reasons if they are reasonable businesspeople too. If they're decent individuals you'll stay on good terms with them. They'll miss you, they'll be "disappointed", but they'll get over it. And don't be surprised if they've already expected you to do this months ago. It is usually obvious to employers when their employees are unhappy or frustrated.
You already know what to do. Now go get it done! Best of luck to you.