2. You are worth more than you currently think you are. Internalize this, know this, that is key. "I am a sole developer for Mobile and Web platforms at this startup in a very small team" --> is a desirable skill in and of itself.
3. Stop working 80 hour weeks, stop working weekends. When the only thing you do has little/no reward, that is what causes burnout.
4. Fill your time with something else that you prioritize above work. Make it hard to find time to work. This both prevents slipping back to 80 hr weeks, and forces your brain to prioritize important things within your work life (like executing and finishing projects).
5. Networking is key. I don't have good advice here as this is a challenge for me also. But -- switch jobs often (every couple years), and be friendly and helpful (within reason) to your co-workers. They're now your network.
My background -- coding since I was 6, now 36 -- but I've shared many of the same feelings.
[1] I am assuming you have the basic financial stability to support this. My apologies if not.