Slow but steady...
I had a dog who was really people and dog shy, and would growl and bark at everyone for a few months. He almost bit my father the second day I had him; my father was trying to force contact. (Dad later felt horrible, but we had both just come off a RIP dog who was well-socialized, and neither of us knew what we were doing!) Early on, I had to be really careful about telling people Buddy was very scared, that they should stay away. His barking and growling at them helped give the message.
Luckily, we walked at a very friendly park, and people were willing to let us trail behind them and their dogs 10 or 15 feet, and eventually we got to the point where people could toss Buddy treats, then let him take treats from their hands. The walking behind really helped Buddy, for some reason - I think he could sniff strange people and dogs and "take them in" through his nose, without ever having to face eye contact or direct approach. We did a LOT of high-reward treating, too - even at first, when things were touchy, from a distance. Buddy began to associate the presence of people with rewards. Now, on a good day, he'll approach people and do a polite "sit," hoping he'll get something from them. He'll probably never be excited to greet large, deep-voiced male strangers, but they don't make him panic, either.
My biggest mistake early on was trying to push Buddy too hard. I had to train myself to watch for signs of stress and learn to remove Buddy from situations before he got overanxious and fearful enough to react. It's really hard to go slowly when everyone else has a "normal" dog and you want one, too! It's really easy to think you've made a giant leap forward, only to find out you've taken two steps back! Aiiee!
The best thing my trainer told me was to show Buddy I'd take care of the stressful situations, so he didn't have to. I'm not sure what they call it - cuing, maybe? - but having Buddy do a specific action instead of what he wanted to do worked to give him that "I'll take care of this" message. Like, when he got scared by bikes, I started walking him off the path, and eventually he learned to walk himself off the path and do a "sit" when he saw a bike. (Same with scary dogs - walk off and do a "down.) I think he thinks it's protective magic.
All this stuff is in the good dog books - McConnell, as suggested, and also "Bones Would Rain from the Sky" by Suzanne Clothier.
Good luck! You can do it!