Chasing the "dream" of having my own business without realising what that actually requires. Then when it got difficult or boring, the next freelancing job is so much easier (and way more lucrative). Back and forth with spending savings on projects that never amounted to anything.

Slowly realised that nowadays, for us techies, there's a much better way of having a good life. Just find the right job (look long and hard), get paid well, hone your skills. I wasted a lot of time.