How to develop sustainable discipline?

I've always been inconsistent.

Since I was a child I've tried plenty of things and rarely finished them. I tried volleyball, basketball, karate, kickboxing, swimming, and didn't want to do any of it after a year or two. My parents forced me to do piano but, as soon as they stopped insisting, I quit too. Even when I thought that I'd found my passion in bodybuilding, that died down after some 4-5 years. Now it's a stretch to get through a half-assed training session without a PT.

I've careened between careers. I got into uni to study Chemistry, but I had no real passion for it. I thought it'd be easy to find a job afterwards, but never questioned if that was something I wanted. The truth is that I was scared of looking for jobs straight after high school, and having to compete against people with better skills and stronger character than I. So I pushed back my rite of passage to the real world and sought refuge in the sheltered, all-forgiving world of upper education, whose rhythm of lectures-study-exam was familiar to me. I managed to graduate but I failed to secure a well-paying job, instead ending up in an uninspiring, low-effort 9-5 position surrounded by other mediocrities. After not even 3 years I'd had enough, and still I stuck to the only script I knew: I went to uni and got another degree. Once again I managed to find a job, this time with way better pay (a 80% pay rise from my 1st job). Still, the technical skills training in this company has been pretty weak for now, so I'll have to take charge of my own development

But how to do that?
I've bought some 3 online courses on Udemy and I haven't finished a single one of them. I've even been toying with the idea of buying another one on Coursera! Not to mention that I have some 4 books on the subject which I've never finished to read. Anything that requires small effort for the long term, I end up failing at. Exams suited me well: lots of stuff to cram into my head in a short period of time, just to vomit it out in a 2h writing spree, then forget about it all. Rinse and repeat. But when it comes to actually learning skills for good and sticking to the process, I am lost. I'm nearly 30 and I still don't know what perseverance means.

I'd like to think that my lack of interest and discipline was caused by porn hacking my dopaminergic system, but I can remember having the same issues well before having been exposed to it. In fact, it might well be that porn is not the cause, but rather the symptom of an untamed, lazy mind which overlooks long-term satisfaction for quick fixes.

I would like to know from those among you who have had the same issue with discipline but have managed to overcome it - how did you do it?
How did you trick yourself to go against your nature?