I feel you on this. I got really emotionally burned like this twice in my career. Now:
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I am extremely careful not get myself in these situations. That means being through in communication and maintaining control of IP until both sides are happy.
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I put a high price tag on my mental wellbeing and realize that a legal fight costs me a lot of pain with lost sleep and stress. That's worth half my salary at least.
Always remember that lawyers WANT you to fight because that's how they make money. They are incentivized to aim as high as possible for you. They win either way, but will win more the more you fight.
Most lawyers won't account for the cost to your mental state that a fight tolls. Sometimes that's worth much more than the money you'd win, especially when you subtract their bills.
Don't fall into this legal battle trap. Focus on your business and work on things that make you happy. Learn your lesson and move on.
You were brilliant enough to create something great before. You'll do it again.
This is excellent advice!