A few words on my direct experience on flexibility trainings, that let me gain something like ten centimeters in one year.
When you work on flexibility, the day after you feel it in your bones, not in your muscles.
Using weights to stretch rather than to gain mass, can be very tiresome for body and impacts even on breathing.
Keeping the stretching positions becomes hard for the spine in two main points, lumbar and dorsal.
The day after a serious training on flexibility you feel your body fixed on the ground, exchanging energy with the pavement or the mattress, while all other joints are open and freely move as they had been disassembled and assembled again.
Your body is open and gently falls on the ground feeling gravity flowing out as if it was a fluid.
When you get up, your body feels more delicate, like having been disassembled and reassembled, but feels fine.
Nutrition is very important, too. When it goes on flexibility keeping light is more effective on joints as it is not the time to build mass, rather to consolidate them while stretching.
About flexibility, do not worry if you feel broken, it is positive. If you give your body the right food and keep it moving, it will heal and enhance. Do not make efforts on a cold body.
When the training is hard, the sleeping is deeper and your bones literally sink in the mattress. The bones more than the muscles. The hardness of training in flexibility is the bones. For this reason, give your body the right food for the bones, too. Your performances will increase.
Last but not least, flexibility training can be done in two ways: with naked feet or gym shoes. Movements are very different. The geometry and the balance completely changes choosing to perform it with or without gym.
Shoes are fundamental with rope jumping and running as both activities are performed in speed, movements are not perfect and seriously impact thousands of time on all the body, rather than with slow controlled isometric movements.
My suggestion, if you are not followed by a trainer, is to be “somehow gentle” until you do not know your body.
Flexibility is about building connecting tissues, not about volume mass gaining, it is harder than it seems. You could get injured and when you get hurt in the joints you do not recover as fast as with the muscles.