This is obviously a mental block. I have had the same thing for all of my smash career, and although it is mostly gone now, I can get stiff hands in tournament. Given some of my improvement was due to learning how to play well with stiff hands, I was also slowly learning how to minimize the amount of times it happened.
The most simple analysis showed me that I only had this problem in tournament, and MM's that I really wanted to win. Friendlies were fine, my movement was much more fluid and less flubs generally. So I soon decided to try to treat all matches as friendlies, and basically adopted a certain kind of mentality where I didn't care if I win or lose.
I avoided the possible negative effects of this mentality by playing the game one stock at a time. So although I didn't really "care" about losing (zero salt, treating it like a friendly), I was still trying to take each stock as quickly and efficiently as possible. So I was playing to win really, I just didn't get nervous or anxious anymore when either my opponent or myself is on last stock, or when I SD'd, flubbed something, or lost 2 stocks in a row. I was able to play on without overanalyzing simple mistakes (I think this is partially because one mistake as Ganon can cost you a stock, lol).
This is basically what helped me. It most certainly won't be the best mental strategy for most other people, but if you can find a way to identify the environment in which you play your best, it could be possible to play as if you are actually in that environment when in fact you're not. Just something to think about.
Play with people and enter tournaments whenever possible. Try your best to focus. Find a way to have fun, and constantly remind yourself to be fluid and not slow down. Don't shield too much. If you're having fun you'll find focusing much easier.
I've never done it, but music obviously helps a lot of people.
Just one take on the situation. Good luck.
