as adam said, it will come. I think we are all hardwired to go into this defensive mode. It's basic human instinct. Boxing, like Freddy used to say, is an unnatural act. Everything is backwards.

One thing you can do is insist to your trainer that he return, and return hard. After a while you'll get used to keeping your eyes open and watching them come at you, and you'll overcome your natural instinct to shell up and bend over and hide. Also try (and this is hard to do, it takes big balls to overcome this fear) actually challenge yourself to bob and weave. Once you find out that you are much harder to hit when you start moving when in trouble, you'll become much less affraid. This is why you see so many guys lay back on the ropes with their hands down when in trouble (which isn't the best habit to get into mind you) but they just start bobbing and weaving and get hit very seldom. They an usually just walk away at that point or hit back. Become a random movement double end bag. They are enough to hit when you know what it's doing, let olane it becoming random movements. Finally, FORCE yourself to Emulate what you see the pros do, or even just the experienced guys in your gym.... you'll quickly find out that many of those things actually work, and you'll gain confidence and try more.