Setting aside the medical issue (which may have something to do with this), if you can't maintain 70 rpm you probably need to improve your cardio fitness. My coach trains us for 90 rpms on flat steady state riding and low 70s rpm for hill climbing. I find this to work well for me, and I can maintain 100 rpm for a while if necessary, though I will eventually start gasping for air. Also if you are pushing big gears at 50 rpm you may eventually hurt your knees.

Again you're only a couple months into this. Work on upping your cadence and just ride. Throw in a few intervals, something like 110 rpm for 30-40 seconds, with breaks in between as needed. as you improve shorten the breaks down to 45 seconds or so.