There are many more knowledgeable folks on this forum than me, and of course, it's hard not to recommend a complete physical, etc.
But, I know that a couple/few years ago when I started riding, I got so that I could ride the flatter stuff, and also do 3-4 hour work shifts doing physical labor, and yet, there were hills I really thought I would never be able to ride up without my HR going to maximum.
I used to laugh when I would read about doing muscle tension workouts on hills to strengthen my legs...they recommended low cadence and low heart rate...just muscular workouts. I would already be at 50 rpm, peddling uphill slowly, and my HR would STILL be totally maxed out.
But, this past weekend, I rode a section of road that used to do that to me, and this weekend, not only did I ride that 5 mile hill happily, with a decent HR, but I kept on going and did another 14 miles of the same climbing...all with a HR no more than about 85% of maximum, and sometimes lower.
Sorry this got long...hope it gives you some hope, if not hard information.
"The best rides are the ones where you bite off much more than you can chew, and live through it." ~ Doug Bradbury