Ride more hills.

Go on rides with clubs like PPTC. The ride descriptions often say H for hilly or MH for moderatly hilly. And riding with a group can push you more than riding solo.

Or if you perfer solo, you can create hilly rides close to DC. North Arlington has good hills, also between Bailley's Crossroads and Annandale. Ride up Mass Ave to the Cathedral, or across Rock Creek a few times. A few months ago I was riding Beach Drive in DC, and to add a few hills I explored every side road, climbing out of the Rock Creek valley and back again 7 or 8 times.

Out of DC, good hills in Loudon, above Gaitherburg, in Prince George County (every time a stream enters the river, you have a 200 descent followed by a 200 steep climb.

I ride PPTC CC and B level rides at a fast C / slow CC pace. I don't even try to keep up with the fast group, but I like being pushed and I take the same hills that they do. Over the past two years, I've gotten much better on hills.
Or the hills are shrinking ..... sometimes it's hard to tell.