It's a narrow path and you can fall off either side. You dont want to get passive. But videos of several of your matches can often show you patterns you habitually fall in to that hurt you so you can avoid them.
The flip side of this is to have one or two things you know you are good at, relative to your overall level. Then at crunch time just do them and nothing else. That feeling you had was choking. You are winning, but you feel so much fear of losing that you are compelled to change what you did to get the lead in the first place. When you feel a choke coming on, you can just run your best play(s) over and over until you win the match, or the opponent makes you stop. And if he starts jumping your best play then any change probably gets you one free point from the surprise.
People tend to overestimate how quickly the opponent can adjust to something. If he is just misreading the spin then maybe so. But if a player is simply lacking a skill, they are not going to learn to do it during a match no matter how many times you go back to it. For example if I play someone who can't push short, and can't bh block a spinny loop after moving in to receive on their fh side, the match is over already and I won. By the same token if somebody has high quality long fast serves to my bh they could serve that all day and I'm dead.
In general to reduce nerves I think it's much better to take the positive approach of saying "This is my best thing and I am going to win with that." Baal's thought process of negative situations and how to avoid them is ideal for training, but not so much for comps, imo.