I've recently stepped up my photography-gear game and bought a few 35mm Film Rangerfinder cameras. They're so iconic, and very cheap these days, that I just decided I had to get my hands on one (2 actually -_- ). Still though, I believe shooting in film will help me learn more about photography fundamentals and build a better workflow.
I acquired a Canonet QL17 GIII 35mm Rangefinder and a Yashica Electro GSN Rangefinder on eBay for about $100 a piece. Both are in pretty good condition. These two cameras are in addition to my main shooter which also has the rangefinder styled viewfinder: Fujifilm X-PRO1.
One thing I have already discovered whilst playing with my cameras is which eye to use in the viewfinder. The viewfinder on Rangefinder cameras is positioned to the top left corner of the camera body. Most people will use their most dominant eye in the viewfinder; usually right handed people will use their right eye.
So it's safe to assume that you're part of most-people (statistically), and you use your right eye. For me, it wasn't even a conscious decision, I've just always picked up my camera and used my right eye. This means that you've been looking through the viewfinder with your right eye, with the left half of your face inclusive of your left eye exposed. You then squint/close your left eye so that you can focus and frame your shot.
After long periods of shooting, I bet your cheek muscles start getting tired from all that squinting. You've been doing it all wrong!