brandy, i've been there. if you know that your FI loves you for YOU, not what you look like, you're ahead of the game. that love will translate into him being there for you and helping you regain a healthy weight, and more importantly, a healthy mind ... one that allows you to love yourself again.
you have had a really very significant gain if there has only been a year and a half between a size 12 and a 20. i too have had very large gains in very short time, but not quite as rapid as yours. you may want to see your doctor just to be sure there isn't a somewhat serious health issue (i.e. thyroid problems, diabetes) or even a drug side effect that is causing such rapid gain. while there, you could also discuss a nutritional/exercise plan that may work better for your lifestyle.
as for dieting ... we all fall off the wagon. going out of town and cheating once or twice shouldn't make you pack it up altogether. don't beat yourself up over it ... just start back up again. lord knows the stresses of planning a wedding can cause us all to overeat once in awhile. we're only human.
and please, if thoughts of picking up your old bulimic ways have been crossing your mind, PLEASE consider counseling. your FI loves you and i'm sure wants you to be healthy inside and out. don't sabotage the long, happy future you could have with the man you love.