I don't work with crystals, but I'd say that if you think this is going to be an ongoing spell, why don't you just dedicate this amethyst to be the crystal you use for this work? I think the altar is a really nice idea. But whatever you decide with the altar, I say the crystals could be reused for a purpose other than combating anxiety if you cleanse them.
P.S. Speaking as someone who also has anxiety, I hope you have considered getting some professional medical help in addition to doing spellwork. No one deserves to deal with that. Take care
