I never drink wine or grape juice ,I stay away from anything containing fructose ,as that is very bad for my digestion, and gives me diarrhea
What should I do for the seder? Is there any alternative? Can I just skip it or drink something else like water or seltzer? I’ve had IBS for more than 40 years and have very little tolerance for additional suffering that could have been possibly avoided.
This sounds difficult, because on one hand we have to perform the mitzva of drinking four cups, but on the other hand it is going to give you an upset stomach. What we drink has to be wine, or at least grape juice, and water or seltzer doesn’t work for the 4 cups on Pesach. You are not alone. The gemora relates a story of one of the Amoraim who would actually get sick from drinking the cups of wine. He did it anyways because he understood that the difficulty will pass and in the meantime he did what Hashem wants him to do. Additionally, he understood that the harder it is to do a mitzva, the greater the reward is, and that it will be well worth the difficulty. On a practical level, for situations such as yours there is an option to dilute the wine or grape juice, which help you keep the difficulty to a minimum. I would advise you to see the suggestions of the Star-K, which is published in their Passover Directory 2021 (abridged edition) https://www.star-k.org/images/passoverdirectory2021.pdf on Pages pg. 130-133 .
Hashem should help you, to fulfill all the mitzvos, and to be able to manage you condition correctly.