OP....I feel your pain. Riding about 50 years now and now have similar pain on my left. I never had any pain in the years past. I have not figured out my problem but one thing that has helped a little is moving the saddle forwards and very slightly lowering it (lower seatpost, not the saddle nose angle). This takes some pressure off the hamstrings and flexors, putting more emphasis on the VL and rest of the quads. It is slight benefit to me. Cost me $400 at a fitter.
I cannot do more than about a 10 minute hard effort before the pain is too much to bear. If I keep the effort down and periodically get off the saddle, I can ride a long distance but cannot ride real hard. Very frustrating.
My next step is to my Chiro who will look a myofascial tension. My Chiro is also a cyclist and has worked the Olympics a few times, so, I trust his cycling specific knowledge. If i were you, I would try to find either a PT or Chiro with cycling or sports expertise. GL