Healthier diet making me feel worse. Anyone else? Tips?

My stomach has been off since I got sick (2.5 years). GI suspects IBS, but not officially diagnosed. Also have POTS and suspected MCAS.

When I was sicker, I ate more processed/convenience food as it was all I could manage. And things with sugar to try to get my weight up (I had no appetite and ended up underweight). I’ve had some improvement with an integrative doctor, so I’ve made my diet healthier. I’m gluten and dairy free and have to avoid high histamine foods and others that trigger symptoms. I’ve been focusing on more fruits and veggies and decreasing sugar. Recently saw a dietitian who recommended I increase fiber and more plant based (I deal with constipation - I would poop everyday, but in little pieces). Also added digestive enzymes, which gave me terrible acid reflux. Recommended low FODMAP, so I’ve tried to reduce, but haven’t fully eliminated.

I felt a little worse (stomach) when initially cooking more and adding more fruits and veggies. Chalked it up to being summer and worse MCAS and POTS symptoms. Now with the dietitians advice, my stomach is a little worse but I find my energy is worse. I got a cookie at a gluten free bakery today and instantly felt better. I objectively know that eating the simpler foods and sugar isn’t good, but in some ways it feels like it is easier to access that energy, while the healthier foods are more work for my body.

I guess this makes sense, given that people who are severe often have to eat simple foods (I’m moderate), but I’m struggling with how to proceed. I can’t find anything that is an easy to digest diet beyond what is meant for stomach issues for a few days (bland diet). I’ve been scared of not getting enough nutrients so I’m trying not to be overly restrictive (feels pretty restrictive already). Maybe try an especially easy to digest FODMAP diet for a few weeks?

Appreciate any thoughts you all have. My integrative doctor feels like improving my gut health is key to further improvement.