You sound like you need a different attitude toward this. You wrote the software and it feels like your baby, but it's not. The baby is not the software, it's the business... and "they" built that. You were a great hire and performed herculean feats of development, but it's their business.

It sounds like you learned and improved a lot while working there. You probably worked crazy hours and have some work/life balance issues. Take a break, relax, VEST, and get ready to start your own baby. Nobody appreciates you more than YOU.

Stop being an employee, you're an entrepreneur, you have the technical chops to get it done, and the tenacity to never give up.