Bonjourno! I haven't looked at this forum for about 5 years and was just flicking through the posts when I came across yours and saw that no one had replied.
I was diagnosed age 35 (am 47 next month) with Thomsen's but never had any symptoms until late 20's. I have always been very sporty - played hockey, rollerskate, gym - but nowaways I find any activity tiring. People say it's because I need to do more but I get exhausted after just half hour in the gym or an hour aerobics class. The next couple of days I will be aching all over and sooo tired. I don't eat pasta as my body doesn't like the high GI effect and try to eat only natural foods and no processed products. My Myotonia gets much worse if I'm stressed or anxious. Yoga is the one thing that really helps and strenthens and also Pilates. I don't get so tired after doing these things.
My husband and I have just started eating a serious amount of fruit - 5-6 bananas a day/oranges/apples etc - mostly blended into a drink, and this has given me much more energy. I always thought this was too much fructose but have now changed my mind as the benefits are many.
I hope you've found advice somewhere. All the best