BCAA's will help a lot. A serving pre workout and can also be sipped during the day between workouts.
Also, use foam rollers on those hip flexors. Rolling really saved me from crippling pain from overuse.
MSM taken at 5-10grams per day also seems to help me greatly.
If you were older, I might suggest /type 1 and 3 collagen, but try the above list first.