I used to have a very similar problem. Maybe I didn't have a pounding heart and shaky voice when someone just disagreed with me, but I have often felt my throat tighten and been unable to say anything when in confrontational situations.
I'm not sure my solutions are applicable to your specific situation, but what worked best for me was handling the confrontational situation instead of running away from it. I realized, that whenever people tried to insult me, or make unreasonable demands of me I would try to be quiet and invisible. At first, I thought this was a more mature method of avoiding conflict. But I eventually realized I was just running away from my fear.
I used to live next to a literal psychopath. He would scream like crazy over the smallest things. And I don't mean scream like a normal person, you could hear an emotionless desire to kill someone in his voice. It was the scariest thing I've ever heard in my life. One time I was just shut the door to my room, not slamming it and he was pounding on my door and screaming rude, racist insults at me. I did nothing. Similar situations happened several times.
And then I decided to stop ignoring it. One day, as he was passing me, I stared at his eyes and never looked away. He never blinked. It was the creepiest eye contact I've ever had. But I felt more confident, because I was able to get over my fear of showing a psychopath I wasn't afraid of him. A few days later he intentionally bumped into me, It was the third time this had happened, but before i'd never done anything. But this time, I yelled, "Don't touch me!" He starts swearing and slamming doors, and luckily nothing worse happened after that.
But I felt so much more confident after that incident. Instead of channeling my adrenaline, and anger into a weak flight response, I finally taught myself I was capable of a fight response.
After standing up to someone as scary as a possible actual psychopath, I feel much more capable of expressing myself to any normal people. Obviously I'm not suggesting you potentially put yourself in danger by doing something similar. But I think you could try to prove to yourself that you are capable of handling confrontational situations.