Hey RT,
Everybody goes through what you describe. It comes in various degrees but every body experiences it.
Here is how I get going again.
First, clean shop. Whatever you have for a shop clean it up. That includes cleaning all of the bits and pieces of the plane that are already built. I'm blessed with a lot of space. All that means is that I can collect a lot more junk while not actively working on a project. Here is a pic of one of two loads of junk going to the dump before I started on the Onex.
While cleaning up the juices will likely start to flow.
Next, get the plans out. Start flipping through the plans from back to front noting all of the work that has already been done. Keep flipping until you find something that needs done. Don't do it right away, just take note. Continue until you know exactly where you left off. It may take several two hour sessions but that will get you back in the mind set and you will likely remember how excited you were when you started. Time spent walking circles around the plane and checking the plans is time well spent.
Next, pick something small that you can do that will move the project along and do it. Remember that 95% of the project is a simple matter of match drill, debur, rivet. The big exceptions to that are wiring and firewall forward. If you are stuck on that, just go ahead and hang the engine (with the tail tied down). If that doesn't get you excited maybe it is time for plan B.
I do know how you feel. I'm in the throws of finishing a Bearhawk project that I've had for 15 years. Taking a break right now but will be walking circles around "Big Bird" again soon.
Good Luck,
Wes