However, there have been a good few humiliating, shamefully weak moments where somebody's demonstrating a technique, brought up his hand to strike the neck, chest, or leg, anything, and I've flinched away from it. One of these people was the sensei, who of course would not exert damaging force to me, a mere newb of three months (this particular example was about three weeks ago) where I've felt like, frankly, a total pussy

It's not about looking hard. I think it's about guts, the courage to take the blow, and the trust that the other person isn't going to damage you. But my issue is with developing this.
I know there are martial artists here in various martial arts that involve strikes, I'm thinking ninjutsu and karate particularly here, but I'm looking for advice from anybody.
As it is I try to take it, and use peer pressure to not succumb to the pain

Anybody got any advice to help me avoid flinching? Of course it's a good reaction to have to flinch, but to train you have to see it from both sides, and it's better to respond than to merely react, I read somewhere. I need to override that reaction in order to be a better training partner, and I think a better martial artist.
Any help much appreciated.