I love the flats (and rollers)!! I love a headwind!! There are many ways to improve your power on the flats:
Power intervals. Do hard efforts of 3 - 5 minutes at a time (building up). If you ride hard and push yourself hard, you can develop your power. Do a mini-time trial where you are pushing yourself to the limit of what you think you can do and just hold that for the interval time. Do "town line" sprint efforts where you go all out to a certain point maybe 500 meters in front of you.
You mention not being able to "keep up" in the wind. Maybe your group riding/pacelining skills need some work also? Sometimes if you aren't on the wheel and a gap opens up, it is hard to get back on. Work on keeping on the wheel. When you rotate off the front, be sure you bring your speed up as you approach the last in line rider so you can get right on the wheel. Work on riding in the sweet spot of the rider in front of you - in a cross wind that is generally a bit off to the side.
Even if you are already talented on the hills, pushing yourself hard on the hills will give you more power on the flats.