I bet it's flatter here than there! OK, depends on where in Texas you live. The Panhandle is probably as flat. Do you have any hills close enough to drive to once a week? Do it.
Last year I started driving down to the Bay Area or up to the Sierras to ride long climbs with some of the others on this forum. There are lots of things that will help, but the best is to actually ride hills once a week. If you have to, find a bridge that you can ride repeats but you need also need a long gradual hill for training. You'll learn how to sit back, relax, spin and keep your heart rate down. If ride repeats on a bridge, be sure to practice sitting and spinning, not just standing and stomping over the top.
From riding the flats for years, one of the big things I learned was to change my expectation of speed. I expected to be rolling along at a pretty good clip all the time because I never climbed anything but a freeway bridge. When faced with 10-20 miles of unremitting UP, I learned to be happy riding very slowly for long periods and I learned how to manage gears to relieve muscle fatigue. Mentally learning how to ride slowly and in a level of constant stress gave me the patience to keep persevering while my legs and cardio got stronger.
I ride in headwinds a lot but it never really taught me the patience and pain tolerance needed for long hills. However, long rides straight in to a headwind for several miles will help and then you'll have the exquisite pleasure (hopefully) of a tailwind home.
Interval training and core strengthening are good compliments.
This may sound like heresy if you live where you can ride outside regularly, but the climbing programs from Spinervals (like the Uphill Grind) will provide an amazing boost to your climbing strength. If you can't actually ride any real hills, even doing the Uphill Grind once a week (no more than two) will be greatly beneficial.
Frends know gud humors when dey is hear it. ~ Da Crockydiles of ZZE.