Osteoarthiritis at the age of 17Watch
I met the GP and she said it is not arthiritis because there is no swelling but....
, try gaining muscle so that the muscle can take over from the joints. hyperextending knees sometimes have a distinctive look that you can google "banana knees" to see. I would say avoid pullups, they take a lot of strength in the muscles and if your tendons, ligaments, joints connective tissue etc feel it instead of the muscle it means you need to strengthen the muscles and learn to fire them before starting. If you're still keen on pullups search scapular depressions, you can do those (letting your shoulder blade shrug up while hanging then tensing the muscle to force the shoulder blades down), you can also do negative pullups, jumping into the top contracted position of the pullup and then controlling the motion down as far as you still have control and repeating. If you have less muscle its good to practice while your doing exercises to focus squeezing the muscle at the top and only going down part way to where you still feel the muscle work and then just working with that till the muscle increases in size and nerve signal so you can feel it as you go down.
More than hypermobility as a description the opposite can also be caused by the muscles not doing the job, so coming away from hypermobility forget the wording and just know that the powers coming from the wrong place regardless of whether it is hyper or hypo mobility. because due to biomechanical reasons, the case is not going to be the same in every part as they create movement in different ways, you could be hypermobile somewhere and lack flexibility elsewhere to compensate.