Paying money back especially money collected from friends and family and not institutions and VCs is not "complicated." Their "investment" never had a chance to make money you projected and you went on to work on someone else's investment. Given these facts I am sure your investors would have preferred to invest in the company where you work now versus the one you left.

It's not family and friends, first of all.

I haven't started working on anything else or for anyone else - I just no longer believe this venture is going to work out and thus have planned to exit. My co-founders don't agree with my assessment.