Okay I no everyone is telling to to use different types of deodorants but I have the same exact problem.(sadly) I found a deodorant that is called "certain-dri". Now ive been using this for over a year now and it's amazing I don't have to worry about sweaty pits anymore. I really hope u use that:) good luck!
Look for antiperspirants that have clinical or near-clinical strength to prevent sweat from staining clothing and putting out an unappealing scent. Consumer Search.com recommends Mitchum Clear Gel antiperspirants as one of the best antiperspirants on the market. Also Degree antiperspirants. Not deodorant There is a difference..
If you're trying out those antiperspirants VickiBankhead suggested and want to avoid sweat stains while trying to find an effective one, there are patches you can purchase that stick to your shirts under your arm-pit and absorb the sweat in place of your shirt. They seem to work quite well :) good luck,
For about 5 or 6 dollars at Walmart, you can buy a deodorant "crystal." All you do is run it under water and then rub it onto your underarms. They work great, have no smell, and are natural. They last about a year or so, well worth the buy.