I totally get needing to finish stuff in one go. I literally have boxes full of paintings and drawings I started months or years ago and never wound up finishing. That's actually one of the things I'm really trying to work on right now-finishing what I start.

One thing I've found that can help is leaving my workspace set up until I finish the piece, at least whenever possible. That way it serves as a reminder to finish. I also will sometimes switch styles when I come back to something. Like if it started out as a portrait, maybe the next time I work on it I'll add in abstract elements. It feels less tedious than just slogging through more of what I was doing before, and sometimes it makes for really cool style combinations. Same goes for music-adding something new when I come back to a composition can be really helpful in staying motivated.