Learning to be assertive, takes time. It's baby steps, to be perfectly honest. Of course, until you come to be in touch, in the moment, with your needs and feelings, being able to assert won't come easy. Many work on 'boundaries'. I italicized because there's oft an initial confusion about what that really means. It's not about laying down the law, it's about knowing your personal limitations/likes/dislikes. Probably the intimate moments aren't the firsr step, albeit a very critical step since that's where dignity will make or break your spirit.
So, after reading further in your thread, I'm wondering about depression and if he's behaved in humiliating ways with you? Depression can go with anxiety at many times. How severely pained do you find yourself, on an emotional level?
Are these panic attacks dredging up painful memories from within your relationship?