I was bloody terrified of all those manoeuvre-type exercises (stalling, steep turning etc) for most of my first 100 hours, and I got terribly airsick whenever I would try them as well, in fact I hated it so much that I damn near gave flying away at one stage, the only thing that stopped me was the fact that I was sure that airline pilots didn't have to do that kind of thing
It does get better though, by the time I was approaching my CPL I was even begging and pleading with my instructor to show me a couple of aerobatics / a quick spin etc at the end of each lesson.
It's just continued exposure to, and increasing your familiarity and comfort level in the aircraft. You can tell yourself in your head that it's safe, but you won't convince your subconscious mind that the plane won't fall out of the sky when you do such-and-such horrible manoeuvre, until you've done it a few dozen times yourself and seen that it actually works.