why do i feel as if i want to gag when food touches the back top of in side my mouth

A lot of the times, it is "all in your mind" in respect to thinking it will make you gag.  Did you have a situation where you had choked on food or had something stuck on the roof of your mouth that caused you to gag?  It could be that some bad experience has made you think that you will gag if food touches the back roof of your mouth.  If you want to try to get over it, you should try to, in little steps, eat something that is easy for you to swallow and try to let it touch the roof of your mouth.  If you can successfully do it without gagging, move on to more solid food.  If you are still struggling, a psychotherapist may have some techniques that can help you through it.