I've had this problem. My solution was when starting a game to play regularly and semi exclusively. I.e. I only have one single player game I'm working on at a time, and I play it multiple times per week to make progress.

And I just keep playing until I beat it. I then may take some time off gaming for a few days or weeks, but will eventually pick up another game and repeat.

This works for me cuz when I don't play consistently and decide to take, say, a one week break from a game, I find I end up not returning. So it's crucial to keep revisiting the game and sensing that progress or it feels like it never ends and drags on and on. There's also things like muscle memory and strategy that if I don't keep fresh at least a few times per week, it makes returning to the game feel like more work rather than just jumping back in.

I don't have diagnosed ADHD so it may not apply to you but that's what worked for me (and continues to work). I've played many dozens of games following this system for some years now.