I sooo feel with you reading this, I've been in a similar situation myself and I know how scary it is
We rescued a grown male and female bulldog who had been spending the past 27 month in a small backyard with only bricks to sleep on. It didn't take long before the male started to bully me around, he would snap at me, push me into corners and stare me out and bark at me. I was so scared I was shaking
We had a dog psychologist working with our case and we ended up giving him our bulldog. He told me though that aggressiveness is one of the easiest things to fix

I have now visited them twice since we gave him up and his behavior is perfect, he is loving and sweet and shows no sign of aggression. So don't give up

I really recommend contacting a good dog trainer, talking to ours made me realize a lot, not only about my dogs but also about myself.
The reason we made the hard decision to give him away was based on many things. My husband was working away for weeks at the time and he was not our only dog. He had major fights with our other bulldog male and it all just got to much for me to sort out by myself. Also I know our dog trainer was madly in love with our dog
I'm sure you'll be able to turn things around

and as people before me have already said, make sure he is not in pain in any way. Bulldogs are tough, just cos he doesn't show any sign of discomfort doesn't mean he is not.
I know this isn't much help but know you are not alone
