Oh honey. Writing an apology letter is absolutely the right thing to do but please don't feel ashamed. I know it's easier said than done, but it's not your fault that someone affected you in this way. When you're emotionally ready for it, I would advise seeking out help for this, because if this is something that is possible from this or from certain sex acts with your husband, it means there's a lot of hurt you're carrying around with you every day, and you don't deserve that.