Before you start in on it identify a milestone you want to hit, write down objectively assessable success criteria, then work on it until you check those boxes. It will be hard but it will feel good and reinforce the behavior. Rinse and repeat. Don’t set yourself up for failure - start with easy milestones and give yourself some wins.

I've got a lot of half-finished side projects that I tinker with and the thing that got me to get them to a usable state was creating a website - I'd had a personal domain for a long time and I decided to put a simple site on it (some information about me and my work etc). I realised quickly that I didn't want to show off half-finished projects so it very quickly motivated me to polish some of my projects and get them across the line.