I am based at Tampa Executive and I fly to North Perry in Hollywood, Fl (Basically Miami) quite often. I also happen to be a psychologist. Your fear is a realistic one and something we all think about. As some of my fellow mooniacs have so eloquently stated, training, equipment and some good old common sense will help.
One of the major factors that get pilots killed is "get home itis." There are just some times that you need to just plan on staying in Miami. Most of the FBO's have deals with local hotels. If you get stuck ask them about it. There are times that YOU WILL JUST BE STUCK over night. I think if you accept that it takes the pressure off a bit.
Another thing to do is to take a longer route. The last time I flew to Miami and wanted to get back to Tampa, the middle of the state looked like a Christmas tree, BUT I could fly west to Naples and North to Tampa. So ALWAYS spend the extra money (gas is expensive in south Florida) and TANK UP. Sure its more gas to take a longer route, but its good practice anyway to carry more gas in shaky weather AND it takes off some of the stress, which is what your concerned about.
Also call the flight briefers. Those guys will help you make decisions (ultimately its your choice) but again sometimes you just need another opinion if for no other reason than to help confirm what your thinking. Again, all in an effort to deal with those fears you mentioned.
Lastly, fly high. I think you have a 231. Get up to 10,500 or 12,500 and it will give you a much better vantage point on what those storms look like. Sure you still want to stay at least 20 miles away, but knowing I am high again makes me FEEL like I have more options and that I am getting the best visual possible.
I applaud you for discussing this openly with us. We all have those fears at one time or another if your a good pilot!
Safe flying!!!!
Hope that helps