There are many different reasons this could be happening. I am not a doctor, but I hope this helps.
Fredda is right on target. Your statement " I've been to the dentist before.." leads me to believe you do not go for routine cleanings. Plaque hardens on your teeth and can not be removed by regular brushing. This harbors bacteria and can lead to bone loss around your teeth, called periodontal disease. This can cause a bad taste and smell.
Is your mouth just peeling after you brush or does it do it all day? See post on peeling mouth, just heard today that crest is removing this ingredient from pro health toothpaste, thank goodness! Try using a different toothpaste, if this doesn't work, You may have a yeast infection in your mouth (thrush), this is a common thing. Your dentist can prescribe lozenges that will clear it up.
Do not be embarrassed to talk to your health professionals. You are paying for their time and expertise, believe me I have seen it all, but it's hard to treat a problem if you don't tell me there is one.
It sounds like you need to see another dentist. You may have infected gums or another dental issue. Also the amount of brushing of the tongue can irritate and even cause an infection. I would first go to the dentist and get a cleaning, as well as any medicated mouth wash to start. You may even need an antibiotic. A family physician can do a culture of the mouth. Gargle with a little salt water may help. Also what kind of tooth paste are you using? Could you have thrush? Do you use a inhaler? Do you have sinus issues? Do you have any bad teeth or pain in your mouth? There are many different reasons this could be happening. I am not a doctor, but I hope this helps.