Sorry you found that out the hard way. High kicks including the roundhouse to the head have their place in self denfense training although it is not in application. You are going to find that as you grow and learn in martial arts that although kicking should be used in moderation, if at all, that it can be an effective tool in a clinch. You can use the your side kicks and front kicks to injure the attackers legs if there is space between you but you are tied up with that person. Where the high kick training comes in is it prepares you to throw powerful low kicks. In my personal experience the only people who I felt had enough power in their kicks at all to warrent ever using them were people who had the ablility to kick beause it develops the muscles and coordination level to kick hard. To give you an sports analogy think of it as a batter warming-up with wieghts on his bat. He is not planning to keep the wieghts on the bat when he steps to the plate, but working his swing with them gives him added power in his baseball swing. Learning high kicks does the same for you low kicking ablilty.
When to kick is a tougher judgement to make in a fight. Rules of thumb of when not to kick would include being on a hill, a slippery surface or wearing tight jean. What you sacrifice when you pick-up a leg to kick is monumental. If sacrifce you mobility. When you pick a leg up the other leg has to stay, realitivly speaking, the the same spot. When you are working hand techniques though you can move around. The tradeoff is power.
Your realization has given you the opportunity reevaulate the kicking repitore that you have and how you could use them effectivly in a fight. Do abandon what you have learned but rather put it into senerio-type training while in the safety of your martial arts school with your friends. For instance, a guy rushes you--practive, in slow motion at first, side kicking his shin bone (which causes his knee to lock up), or front kicking the front of his thighs. If you get together with your friends you will figure a lot out and hopefully your instructor or instructors will be open minded to this.
On a last note kicking is probably not the best way to stop a bulls rush, but look at other stratgies like turning when he grabs or sprawling if you are on surface that allows that. If you learn to think backward, from the opponets point of view, i.e. what gave him conifedence in the bulls rush than you can preformulate stategies that will work.