Since I updated to Scrivener 3.5.1 for macOS my trackpad will not not go further than the last 3-5 letters on any line in Scrivener. I did check that it wasn’t a problem on other writing such as MS Word and Pages. It works just fine on them. ALSO I can’t highlight from the end of a line to the left, but can highlight if I start the highlight from left to right if I start with more than 3-5 letters. It isn’t a world shaking problem but is a bit of a PITA. 
I cleared everything I could think of on my computer and then restarted the computer and low and behold Scrivener is working like a champ now including those last few letters at the ends of everyline. I doubt it had anything to do with the trackpad since the same thing happened with the mouse BUT all is well and this old woman is a happy camper.
Probably the macOS software that Trackpad and Mouse shares to give you the ability to select text. Re-booting often fixes these sorts of things, especially if Blue Tooth involved.
Glad you are back to writing.