I gag a lot when i brush the back of my tongue..and often with the taste of toothpaste too. what can i do to prevent from gagging and throwing up?

Reflex: Part of it could be reflex. Part could be the toothpaste you are using. Try a different brand that may be more appealing to your taste, and don't be so aggressive towards the back of your tongue. No need to vomit at the thought of brushing your teeth.

Modify: Brush you tongue in sweeping “gentle” action. Try a different toothpaste until you find one that you like. The action of brushing the teeth with a soft brush is much more of an important factor than the exact type of toothpaste.

Gagging is okay: You can try to use tongue scraper, but gagging when brushing tongue happens often. Sometimes, in a dental office, spraying topical anesthetic on the palate reduces gagging reflex.

It's only normal!: Don't brush the back of your tongue!

Modify technique: It is great that you are brushing your tongue and removing the bacteria living there. It is common to gag if you get back too far. Try a tongue scraper or don't use toothpaste back there. Modify your technique and dot't goo back as far. Good luck.