When you are under pressure, there isn't really conscious control of your actions. Your brain really hammers out what you are going to do before you become aware of it. If the gears in your brain think that staying in the pocket is suicidal and do not trust your training, you will not be able to actualize it subconsciously. Different people have different requirements for what it takes to actualize the training.
A lot of people talk about sparring, or even fighting, as if they are talking about juggling or skipping rope - that it is simply coordination and that you can get better just by practicing the motion. That's not enough in my opinion. I know it isn't enough for me.
I have to build up the correct form by drilling techniques full contact (once I have built up the coordination aspect). I need people aiming for my face and hitting me in the face (or leg or whatever) if I fail. Sometimes to prove that they are willing, I stand their and take a hit, showing I really was getting out of the way. The deep gears of your mind need to believe in what you are doing. If the deep gears buy into faith, this isn't that hard. If you have to see it to believe it, getting better at fighting is a lot more work.
Trust is a big issue. While picking things up from drills is hard, anything I end up doing on a fluke in hard sparring I can generally repeat afterwards under pressure without practice. Once it happens, it is in the bag.