All the best, Mudgel! Hope everything goes well.
As for the OP -- happens to me all the time. Two things I have done that help:
1) I work on more than one project at a time, all the time, and I try to mix up the genre/styles as much as I can, so that when I switch back and forth its refreshing and I don't accidentally copy ideas over from one project to the other. So, I'll have a classic, old school orchestral piece going on, alongside a sketch of Celtic ethereal folkishness, and/or maybe a hybrid, synth heavy trailer type thing.
2) Since Christmas, I have been trying to write and record music fast as I can, at least for the first draft. It gets the initial composition portion out of the way so that I get down to crafting the polished version. It cuts down on the overall time a project takes, and so far hasn't affected how the music turns out - to my great surprise. This is a total change of approach for me, and has been really helpful. I can be very painstaking and deliberate in what I do, and I'm a perfectionist -- and my own worst critic. So, I can get really easily bogged down in a project and grow to hate it in no time flat. I've headed that tendency off at the pass, to a certain degree, by just writing music in a white heat and ignoring mistakes. The first product is usually complete crap. But surprisingly the general shape and vision of what I was trying to create is usually there in some form, along with a few happy accidents and unexpected pleasures. And you can build on that pretty easily, I have found.
All this helps a fair amount. But there are still days I have to close a project up, and go do something else -- like grab a can of cider and go watch The Orville for the rest of the evening, or something.