Hello again and thank you for your response.
In case of decayed tooth the unpleasant taste can affect the entire mouth and thus even tongue.As you breathe you can feel it coming from your nose too.Its all inter-related.
Please don't feel embarrassed about going to a dentist as this will worsen the situation more for you and also for your tooth as decay/caries is an irreversible process and it will continue to undermine the tooth surface which demands professional help!I am sure your dentist will comfort you well and provide you the correct treatment after clinically examining your tooth.Any decay(the black surface) has to be removed and the tooth needs a re-filling and a build up as it is half broken.
Receding gum's can be due to over-brushing which will encourage periodontal
/gum disease ,which also contributes to the bad taste you experience in your mouth.
Caries tooth + periodontal disease= more bacteria= more unpleasent taste.
At this point home remedies and self help(which i assume you have done it all!,) will not help you much and may be you'll feel the taste is gone as soon as you brushed or flossed but it will tend to come back as the source of the smell is not yet treated.
The raised white dots are the normal anatomy of the tongue the papillae-fungiform and at the extreme back are the circumvallate papilla which are about to 8-10 in number.These contain taste buds and are not related to the bad taste you are having in your mouth,probably you are noticing them now.
I hope this helps!
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