Roman Chair: One sees many devices called Roman Chairs, but the above is what I think of and use. This is a specific hip flexor exercise. Works like a charm. Note the height of his knees at the top of the exercise. If you can do 20 of those, you're OK. One can also hold a dumbbell between one's feet for extra credit. "Failure" for me is when I can't get my knees higher than my hands - I stop.

But don't be that harsh on yourself. Do what you can, and work on getting better at it.