There are some people that say 'always better to have a plan B'. Others instead say 'never have a plan b so you can go all in with plan A so you can put all you've got to it'. I don't subscribe to either school of thought. Both have their pros and cons.
Have a plan B
My suggestion would be to always have a plan B in the back of your mind but to act as you don't.
This way you get the benefits of both systems without their cons.
You can still go all in with plan A even while knowing that if that doesn't work out you have a safety net. Not having a safety net at all might work with some people, but it is unnecessarily reckless.
Committing to your work should not be based on the fact that what you're doing is the only avaialble option you have before failing. You should commit because it's the right thing to do and what you want to do.
Don't work on a side project that feels like a burden. If you grown apart from your project is fine, it happens. Consider selling it or transitioning it to another indie developer that can give it the attention and time it needs.