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I feel like a failure

I am doing FB strong for the 2nd time (not in a row) and just did the day 3 core workout. I feel like a total failure because I just can't manage a straight leg side hip raise, I always go off my knees, same with a T stabilization I have been able to do both in the past, and I know fitness and strength vary day to day, but it's such a kick in the gut at times. Does anyone else ever have this happen to them?