…concentrate on contracting your glute on the trailing leg to help stretch your hip flexor.
Try 2-3 sets of 8-12 reps per side.
Stand with your feet approximately double shoulder-width apart. Shift your weight to one side and drop your hips down and back, keeping your knee over your toes and your weight over your bent leg. Pause for a 2-count, return to the starting position and repeat to the other side. Aim for two or three sets of 8-12 reps per side.