yeah this happens to pretty much everyone. The best thing you can do is try to work efficiently and quickly so you don't get tired of your own song from hearing it too many times. Making sure your library is organized and dedicating time to just doing sound design (while not arranging) can help a lot with this.. for example, having producing sessions where all you are doing is making patches, racks, processing drums, making risers and transition fx etc. and compiling all of those into an organized library while not worrying about an arrangement or any track in particular.. then when you are ready to compose and arrange a song, you have a bunch of tools and sounds at your fingertips and you won't derail your concentration by going back and forth between sound design and composition.

however, personally i still will do sound design stuff while composing just because i like to... but it definitely does make my process less efficient than it could be. When i reach a point where i find myself getting tired of a tune, i will usually just take a break from it for a few days and work on something else for a bit so i can come back to it with a fresh perspective. Sometimes when i do this i end up not finishing things because i decide its not good enough, but sometimes a break is all i need and the "magic" comes back

just keep at it though! this part is difficult for sure, but the more you do it the easier it will get, nothing can replace experience