Congratulations, for having the courage to take the jump!
For heart racing; salt always helped me, but it may be the vit B. Sometimes, I cant take it; it may cause the racing heart; has something to do with adrenal fatigue, but I dont know the technicals of it. I have searched, but could not find it, but tested it for many years. Typically high pulse, pounding?
Quit the vit B for a week, then you will know.
4-6oo mcg of selenium is on the high side. Depends on where you live. if US; it is high, the soil is higher there than in Europe.
just check it is the methionine form, but probably if it is yeast free. For 125 mg of Iodine I would say 200 mcg is good enough. If thyroid pain, adjust with more selenium as you go upward in dose, if you do:)
Also you may try taking some gelatine / glycine, if it causes insomnia.