If you live in the USA, Vince's mouthwash is what is used for thrush. It's a pink or white powder in a rectangular jar.
If you are a vegan, you might not be getting the vitamins you need. I agree with Jemma 116 - you need to have your B vitamin levels checked by a doctor and lab. Beri Beri is what a lack of vitamin B used to be called. It was more common in people who ate mostly white potatoes or white rice.
Wishing you the best
I have copied and pasted the link below that you may find of interest.
http://www.livestrong.com/article/504289-vitamin-deficiencies-and-cracked-tongues/
It may be a good idea for you to get a blood test done to check your Vitamin B complex done (especially B12) to check the levels and to see if you are deficient in any other vitamins and minerals like iron.
Taking antibiotics can bring on the symptoms of Thrush and a swab taken from your tongue would confirm whether you have a bacterial or fungal problem. The treatment for oral thrush is Daktarin for oral use which can be bought from the pharmacy without a prescription. Fluconazole is a one dose oral capsule for Thrush and can also be purchased from the pharmacy without a prescription, this will treat genital fungal problems.
You can help yourself too by eating natural live yogurt and avoiding highly spiced and sharp foods and drinks. Also drink more water and try a mouth wash using salty water or dilute a teaspoonful of Bicarbonate of Soda in a glass of water and use that to swish around in your mouth to see if that will help.
Another condition that I am aware of that can cause this is Lichen.
Best wishes.
I don't really even know why your doctor cannot find out what it is, I suggest you try to have check on a witch doctor...some decease will cure from witch doctor if a real doctor cannot cure it...that is only my suggestion, nothing to loss if you try...Regards