I do know exactly how you feel. Having a reactive collie dumped on my doorstep literally I have also been Billy no mates. Made worse as people know I am a dog behavourist and should have perfectly behaved dogs at all time! You also have to learn to ignore when people with no experience of reactive dogs know exactly the right thing to do and usually involves dominating the dog or hitting with a newspaper or alpha rolling!!!!!!!!
It is embarrassing when people turn away when they see you coming and tut loudly as your dog barks etc. However there is hope. My man is know ok! I know what he can and can not do - we go to agility together, we do working trials together and he would do anything for me. He is the most loyal dog I have every had (sorry fostered OH does not know he is here to stay!) Some things are just too much for him and we are still working on his reaction in some situations but we have got to a place where we can work with this.
First it is not your fault and owners who work hard with difficult dogs have to be very special people and I know you are 
Ok what to do about it on a daily basis?
Does he like toys? If he likes retrieving then you can get him his off lead exercise by playing ball in a very wide open area where you can see for a long way off.
He can then be walked on lead in more dense areas where you can not see dogs around.
Also if you know anyone with private land you could let him run off there off lead safe in the knowledge that no dogs will be about. (not always possible though)
Training wise you need to look at Behaviour changing, so no punishment, no shouting. Your dog needs to change his reaction when he sees other dogs. It can take time but it WILL work.
BAT is fantastic link here
also try and get a copy of Stop here as it does sound to me like an over excited chase response.
The people who can help you are APDT if they can't whereabouts are you I should be able to put you in touch with a trainer who uses BAT.
Also try to think of what you can do with your dog - he is not all bad, does he give great cuddles, is he the funniest dog when playing with toys etc. It gets very easy to concentrate on his one fault (although mega) and forget that he is still your dog who loves all that you do for him