I have been having a problem lately with the left button. I can’t always highlight and copy or when playing facebook games it doesn’t always work. I changed the settings (win 10) to use the right button instead of the left and it works but is awkward.
Well, maybe the mouse is sick — just not ready for the grave. If you have found a replacement for the one that you were having problems with, you might try operating on it and maybe fixing it. If you don’t succeed you haven’t really lost anything. And, if you do fix it then you have a spare, and some satisfaction.
It sounds like whatever the contacts are in the mouse for that left button, they probably have gotten dirty. Maybe just picked up some lint. If you can get it open, then maybe a spray with some WD-40, or air from an air can might do it. I like high-test Isopropyl alcohol (used to buy 99% until that was taken off the market — but the 91% will work). WD-40 leaves a film and doesn’t like some plastics — Isopropl alcohol is safer.