You said you've been back on the bike for just two months? In that amount of time you're doing fine. I'm not qualified to comment on your medical issues and its effects, but I suggest you include some type of hi intensity intervals at least once a week in your riding. Find a hill to train on. I have one near me that is 4-6%, about 1.2 miles long. I'll do 3 intervals on the way up, come back down, then do a steady state climb back up maintaining low 70s cadence in a gear that makes me work. I use a heart rate monitor so I can learn how long I can sustain an effort at a given rate.