The ones that failed tended to be 'passion projects' where I was led by what I wanted to do, or what seemed cool, and became obsessed with 'finishing' them to a high standard rather than releasing (or abandoning) them. One failure was a puzzle game for early iPhone (iOS 5 I think) - it took 6 months, but ultimately was far more fun to build than to play. Looking bad, I was kidding myself - it looked average at best, and there were far better people and games out there than the one I was building. I learned some fun things but I probably should have stopped after a couple of months, whereas I think I threw 6 months at that one.

The success stories were the ones where I was led more by a sense of market demand - e.g. there was a problem that needed solving, and I had some awareness that existing solutions either did not exist, or weren't a good fit.

I built one of the very first apps in the transport tech space and that was a big success, both in terms of adoption and profitability. That was a combination of skill and luck - i.e. 'right place right time'.

While I have now had 2-3 'success stories' in terms of products that took off / gained significant adoption, I must say I haven't fully learned my lesson and I do still have the same obsessive tendencies! Coding is addictive huh. I recently spent 3 months building a full stack React app that I just did because I wanted to "finish" it, and I'm not really sure if there is a market for it. But I had the time available and I think the older I get, the more I can self-police and know when it's time to step away.

Also, if you only spend a small amount of time on something, even if it doesn't get used, as long as you've learned new skills or experiences, then it might arguably have been worth it.