Congrats on the fiddle playing… Hope that goes well for you!
There’s no rules, it’s not a job and no one is putting pressure on us (other than ourselves). You do whatever makes you happy and keep it fun and enjoyable.
To answer your question a little more specifically, I wouldn’t work on a lesson so much that you get burnt out on it. I like to do as Archie said and work on a few at a time. I think that makes it much more interesting and just plain fun and keeps you from the burn out. Also, playing and learning other songs will help with that specific tune you may have been working on for so long. Just playing something will even help.
Mix it up, play things you know well for enjoyment, work on things that are tougher than you think you can handle to keep yourself challenged (this is important). It’s the only way to keep from getting stuck on plateaus. Most of all, always make sure it brings you joy!