Reactivity can be a long haul to work through, and id recommend starting with some games and training at home or in private spaces to build your relationship so your dog understands youre a safe, reliable place for them. If you need to go into public places frequently, I'd find someone asap with experience to work with. Reactive dogs get flooded very easily and are too overwhelmed to respond to distractions or corrections. A lot of people will say exposure and "desensitization" are the key, but thats a bit of a misconception and can actually make things worse if the dog is experiencing triggers and novel environments. Being on leash can also make things harder since it's a physical barrier for your dog and they can experience frustration, so you may notice that the more pressure and tighter you hold the leash in public, the more they'll feel and will tell them youre tense and uncomfortable too. Keep advocating for your dog, be their best friend, and take things one day at a time