Very limited leg abduction

[soulnafein]

Hello,
I have been practicing martial arts constantly for two years and I have a decent overall stretching although a problem that limits me a lot is my stiff side leg movements. I can reach a very good height with front leg raises but I can't even reach 45 degrees with side leg raises and splits.
My concern is that I don't feel stretching in the groin area but I feel a pain on the side of the leg and knee.
I have noticed that when I stretch with someone sit on the floor (e.g. he pushes my leg apart) i have a very limited opening and pain on the side of the knee but after 2-3 minutes I can open much more and slowly the pain moves towards the groins as I would expect.
Could you help me to understand how to improve this situation?

Thanks.