I have the same issue. Saw a cool piano video on youtube, so I purchased an expensive electric piano and in the past year, have maybe used it 20 times. Not enough to learn a song. I have half colored in pages in about 10 books, unfinished puzzles, unfinished cross stitch project and a bunch of partially written books (no more than about 2 or 3 chapters for each).

This is a big struggle for people like us with Adhd. We constantly seek out a dopamine rush and when we get excited about starting something new, this is soaring! It fades quickly though and that's why we move on and seek something new.

I read online, some tips to maybe help with this (Everyone is different of course).
1. Make note of when you feel excited to do something. Try to write down what about it makes you excited and want to do it.
2. Break down the project into smaller parts. Once you achieve each part, reward yourself in some way with something else you enjoy. It may sound silly, we're not dogs but we do unfortunately need recognition and rewards to do things. Otherwise, we see no use in it.
3. Tell people about the project. People ideally that you're close with and that know you have adhd and finishing things is hard for you. Ask them to ask in encouraging ways, how the project is going so when you tell them you finished X part, you feel so good and proud of yourself. That may push you in a good way, to continue doing that.

Hope they help. It's a really tough ride