Hi Rabbi, I am a terrible faster, I am unwell during the fast, after the fast and for 1-2 days after the fast. I am still feeling terrible even now, 28hrs after the fast has ended. It affects my work, and I’m afraid of potential Shalom Bayis issues for being in a lowered mood. Other than 9Av and Yom Kippur, is there room for leniency for the minor fasts, especially if they are nidchei to not fast?

Thank you for your question, You do not have to fast on the minor fasts (even if they are not nidchei). You didn't describe the problem in detail so I don’t know if you can be lenient on Tisha beav (however Tisha beav nidchei is like a minor fast). Sources: see caf hachaim (150;6), aruch hashulchan (150;1), igrot moshe orach chaim (part 4; 114) All the best!