Totally know how you feel. I had to give up feeding DS when he was about 11 weeks as I was in the same situation and DS was lactose intolerant and my dairy addiction didnt help. I felt awfully guilty but as soon as I compared a photo of DS of 2 weeks after going onto formula the guilt just went. His face was full, his colour improved and most of all he was more alert and happy.
As for what your boobs do - Mine only felt really really full about 2 days after stopping and when I was in the shower it just started to gush out. I expressed for about 5 minutes to give me some relief and avoid mastitus and they didnt fill up again. Would get the occassional tiny spot of milk in the morning if I was laying on my front, but that it is. Within a week you'd never have know I had been BFing in the 1st place.
Don't feel guilty - you've given LO a good start and now he needs that little extra. Enjoy the happy baby that will come with the formula as it is obviously what LO needs.
XxXx
P.s. with next baby you may not have the same problem so don't be put off for next time