Folate / B12 / methyl donor intolerance

Any ideas at all on what can trigger a person to not be able to take folate or B12 without really bad symptoms? I have had 3 horrible years (undermedicated on thyroid bad and developed SIBO, leaky gut, high homocysteine, histamine intolerance). I used to be able to take folate but in the last couple years I cannot take ANY methyl donors or else I really feel like I'm going to die (extreme head sensations, extreme fatigue, aches, tingling - I can't even engage). I can't even eat folate fortified foods! I've also discovered I am undermethylating and not a good detoxer. Am working with a good new functional doc who HAS helped me make progress (I think mostly better because I'm finally back on a little thyroid med). Is this methylation-cycle related?

MTHFR A1298C ++
MTHFS - ST20 ++
COMT ++ (14 of them)
FUT2 - all 3 ++ so I am a nonsecretor