Aww I'm sorry to hear you're conflicted like that. Making these kinds of decisions are tough, but making the right decision is worth it. I guess all I have to say here is do what makes YOU happy. Group B seems like an amazing group with how you described them, and you obviously don't care for how Group A is. If you pick to be miserable in Group A, that takes away the whole point of roleplaying. It's to have fun, not drag yourself on and hate doing it. Roleplaying is suppose to be an experience that takes you to another place an lets you escape, not a place you fear to be. I personally think that Group B deserves you back with them, as YOURSELF should come first and foremost in roleplaying. You have the right to be HAPPY when you roleplay. I can understand how being with a cruel group is uncomfortable. If you wrote up or said a nice and sincere apology/resignation to them- I'm sure they'd understand. They are people. Plus, if that roleplay dies, it's not the end of those peoples worlds. FH will live on and so will they. If you want to continue your roleplaying and be happy I would go to Group B, for the simple reason that it makes you love to roleplay.
As many say, the final decision is up to you. I hope I aided you in some way. I understand how it feels to be stuck between a rock and a hard place. Best of luck to you~