I understand how you feel, my hip flexors use to always want to do a majority of the work. I found that changing the angles slightly, maybe not going as far out or letting my leg drop as low, has really helped me to concentrate more on the lower abs and build the strength I need so i am not relying on my Hip Flexors to get through Abs. A good way to find the lower ab muscles you need for the Abdominal work is to sit up straight in a chair, place your fingers on your low abs (so you can feel the activation), while keeping your knees bent try lifting one knee as high as you can into the chest with out feeling it in you Hip Flexors, you should feel your lower abs activating under your finger tips, switch legs, then try straightening your leg and lifting, maybe then try and take it to the side (if you feel too much hip flexor here then maybe keep your legs slightly bent in the ab section or maybe not take it our as far). Your hip Flexors will obviously aid you in bringing your knee up but this will help you feel where your abs activate, give you an idea of your range of Motion, and where you can send your energy too in the Ab routine. I hope this Helps!!!