The first thing to get straight is that it's NOT an irrational fear. You probably are however, over analyzing the situation.
I was safety pilot in a pal's C310, and I flew into cloud. He suddenly shouted Down! Grabbed the controls and got himself into clear air. He went on to a CPL IR etc for private use and flew for the next 35 years.
You know inside that your life depends on a few gyros. Perhaps even just one. It's a nutty situation to be in. A mile above the ground with a tiny wheel keeping you from spinning out of the cloud. People who don't have a spell of questioning this state are perhaps lacking in other ways. However, when all is said and done, you'll have to master these feelings.
I had to give myself a good talking to after several nights of bad storms. No radar at 18k and getting thrashed night after night. After one really bad one, I almost gave up. That's when I had to make a decision that whenever things got crappy, I'd grab the bull by the horns and master it. No time for analysis, and not time for fear. There's a job to do and others will be relying on you...that kind of talking to.
I was a training FO for a year or so, and sat with a succession of new captains most first time on jet transport aircraft. Most were fine, but there were a few who really lost it for a few minutes here and there. They all but one went on to long careers.