That yellow layer is called as fibrinopurulent membrane and is more or less normal. The bigger issue from your description seems to be the size of the socket which should have closed up substantially by now. Maintenance of oral hygiene by proper rinsing and tooth brushing is the least that you can do. Extending the course of antibiotics is not going to solve this problem. The best action would be to visit the dentist for a proper examination and evaluation of the current clinical situation. Regards.