I think I'd probably finish a lot more projects if I wasn't a professional software developer -- I don't want to work in my free time as well.
What has helped me is to get better about goal setting. For me this means having some large goal that is easily associated with personal desire. Then decompose that into smaller goals. These smaller ones are one step removed from the base desire, and as such need a little effort to figure out how to attach the goal to desire.
I keep breaking down the goals until they are a size that I can accomplish in a day. Often that's too small a timeline but that's what I aim for.
At each step of the break down, I make an effort to attach that goal to personal desire. Whether the desire is related to the overarching desire for the top goal doesn't matter so much, but it does have to have a desire component.
Sometimes the desire attached to a goal is to end the day feeling like I have made progress. That's it. The whole desire for that goal is in the achievement.
And frankly, achievement is a huge motivator for me. If I feel like I am spinning my wheels, it makes me what to stop and do something that isn't wasting my time.
Breaking the goals down into smaller goals that work on shorter timelines allows you to get that dopamine hit from achievement. You can end the day feeling pleased and relaxed. And you can wake up excited that you have an achievable goal to work on for that day.
Simple life hack: Next time you're planning on starting a project like that, declare that the goal in advance.
If you're finishing projects at work, you know you could finish, if you wanted to, but whether you "finish" a project should be subject to cost/benefits analysis just like anything else. Your $COOL_PROJECT isn't a failure if it doesn't attain 15,000 stars on GitHub, unless you chose to make that a goal.
There so many things I would do before I'd do something unrewarding.
I've also found that how intellectually rewarding my day job is (and how much agency I have) has a direct affect on my hobby development. I have a lot of creative freedom and interesting projects at work right now so I care less about coding in my spare time. I only have so much free intellectual energy to devote to something.