newbie here

Hi i'm new here and figured someone might be able to help.. i'm 17 and im from Long Island... for as long as i can remember i've been suffering from Diarrhea....about 2 years ago i started going to a GI Dr. and she told me to just increase the amount of fiber i eat... i tried that and still am... i have been eating raisin bran every morning for over 2 years now... if i eat too much i get gassy... not enough and i get diarrhea.... ive tried to eat whole wheat bread at lunch but thats not always possible.. and though that helps with the diarrhea... i get very gassy with too much fiber... ive read that people should normally have 40 grams per day... i have rasin bran which is only 7 or 8 grams... if i eat too much i get gassy... for awhile eating raisin bran every morning helped but i still get diarrhea... now a few years later ive had all sorts of tests and my dr. cannot figure out what is wrong with me... she is thinking IBS.... she put me on Immodium for about 3 weeks.. and told me to eat more rice... i'm having problems with the dosage.... 1 pill and i get constipated.... a half doesnt seem to be enough.... i've tried 3/4th and thats only sometimes enough... shes putting me on small doses of Zoloft but she doesnt know if it will help at all... i cant stand this anymore... it is ruining my life!! i cannot do anything without worrying about getting diarrhea... i start my freshman year of college in september and its going to be even harder because its not so easy to run home and use the bathroom especially with a 30+ minute drive.... please can someone give me some advice? and i guess talking to other people who suffer will be comforting because its hard to talk about it with other people who do not really understand it... thanks