I'll start by saying that the best thing you can do for your technique, any technique, is to practice it over and over and over again. An old instructor of mine used to tell me: "there are some techniques you need to do a million times, and the rest you need to do two million."
Besides that, though, I will say that it's not flexibility you need to develop as much as the acrobatic athleticism that most strikers have. (there are very, very few exceptions, and they're fat heavyweights)
In order to do that, I would suggest practicing your striking alot with alot of focus on your kicks, as far as conventional methodology.
However, if you want to be conventional, there are a few excersises I really, really suggest for developing those kinds of things, and they will help you as much with your ground game as with your standup.
Yoga is probably the most underrated form of excersise in athletics. I do vikrum yoga to clean out my body, to cut weight and generally to keep my flexibility up. It will also help with your balance, which may be another reason your body isn't working the kicks very well.
Free-running or parkour can help you develop you strength and cardio as well, and will definitely help develop the raw athleticism that you need.
Gymnastics is something that is an invaluable skill that you don't see alot of MMA guys talk about, but alot of fighters have done it in the past. Gymnasts are some of the most fit athletes as well as some of the pound-for-pound strongest. I have some experience with gymnastics and, in fact, one of the instructors at my BJJ school was my gymnastics teacher.
Athleticism and flexibility are directly related, and that's something that I really think will solve your problem. Definitely use the conventional method of training and stretching regularly, but you might also want to look into the non-conventional methods.
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