I too am in a similar situation, where I am building a niche product - partly for my own benefit, partly for learning, but mostly with the idea of selling it as a commercial product.

I have plenty of worries about it - will the product sell at all, is the product too niche so I'll have sales but not enough to make it full time, am I barking up the wrong tree and there is already an open source free alternative that I've somehow missed, what if nobody likes it? All sorts of stuff, some warranted, and some just the usual fear of making something and putting it out there.

With that being said I do consider a big portion of success being luck, as any one lucky event could catapult you to riches, and any unlucky event could ruin any chance of that happening, but in the end you have to take a risk and put yourself out there for the lucky events to happen.

But as with all risky things you have to be prepared for it all to go to shit, and then have enough of a support network which will help you get back onto your feet.

I genuinely hope that other people have some more concrete advice here or even war stories to tell.