Why is my tongue thickly coated?

For as long as i can remember ive had a very thick dirty yellowish coating on my tongue that mainly gathers in the middle and back i also have a crack in the middle this is really concerning me as i also suffer with alot of phlegm after eating food and stomach problems not to mention dry skin and hair loss im always drinking lots of water everyday so i dont think it could be dehydration unless im not absorbing the water i drink as i pee almost straight after and i dont think it is thrush as it doesnt look anything like that.Im not sure what to do its affecting my life so much :(