Are you pushing to much?
As your ability improves, measured by an increase in speed on the flats, the extra energy and leg strength required to hold that pace could mean you have less endurance for the hills.
If you slowed the pace down to 18 - 20 mph, would the hills become easier?
I'm working with an ex-cat 2 racer and we were just talking about lactate levels this week. He said there is no magic formula, you just have to keep pushing your body to handle the load. He rode 7 days a week.....intervals after intervals, long distance rides at speed and motor pacing at 40 mph. He said learning to ride through the pain was key to improvement.
At our age, not sure how much pain I'm willing to endure to gain more speed or climb hills faster. But it does sound like you'll need train even harder if you want climb hills with less effort and minimize the pain.