If the dog has fleas she needs a bath to eliminate them. It can be done at home, but can also be done by a groomer, if you feel that doing it at home might make her even more withdrawn. There are chewable medications for fleas, such as Nexguard, which can be given as treats so that there is no need to apply a liquid on to the dog's skin. Speak to your vet about it.
Getting hold of the dog has no easy solution. You can try putting treats in her cage if you have one and letting her get used to that. If she came from a mill, or some such environment, the cage should make her feel safe.
It takes a while to get a very insecure dog to trust. You could try putting treats closer to you, such as when you're sitting on a couch, for example, and let her get them at her own time. The more you do it and the closer you place them to you, the more likely she will start to come closer. As you know, consistent repetition is the way most dogs are trained. Most will respond, although it can take a while. Some, unfortunately, will not. Good luck.