I have the same problem. Once I get some sense of understanding the overall skill or problem I lose interest? Novice skill is definitely a worse thing to have and getting better takes a lot of consistent work and time.

How do I fix this? It seems like a motivation problem, possibly some sort of craving for novelty perhaps (I don't want to say ADHD but sounds like it)?

ADHD for me. It became such a problem that I saught help from the nurse practitioner at my Uni and got a diagnosis (at 27)

What techniques do you use now to cope? I'm going to go get a diagnosis as well, I think I have enough anecdotal evidence to talk to a medical professional.

It has a lot of comorbities with other things, but it's worthwhile to see someone. I haven't yet found a way to reconcile my need for a job (particularly one in programming) with my symptoms. That said, concerta seems to help a bit under certain conditions. I also afixed my house key to my pants. I have learned to enjoy the bits that make me outgoing and end up letting me meet tons of people

You can fix that by raising the bar for which you feel you are "remotely competent". If you consider anyone below world class to not be remotely competent then you'll be world class before you give up.

Alternatively, keep piling on new ways of doing something, like raising the difficulty slider in a video game to keep it challenging.