Painful!
Washing your mouth out with salt water will kill a majority of the germs, that'll help!
They usually show up when your taking the least care of your mouth (not keeping it clean enough i.e. brushing, flossing, and Listerine)
You just have to let it be and do a better job brushing, maybe morning and night brushing is necessary
The only thing you can do to cure mouth ulcers is to brush your teeth after each meal until the ulcers go away. Rinsing your mouth with warm salty water a couple times a day works as well.
But to alleviate the pain, you can use Bonjella if it's available to you. It's a clear, orally applied analgesic gel that tastes like liquorice that you rub on the affected area, but you can probably get similar products if it isnt available. It sorta numbs the pain.
Prevention: Since ulcers are caused by bacteria, regular dental hygiene should prevent new ulcers from forming. Brushing your teeth twice a day, flossing and using a non-alcoholic mouthwash once a day is adaquate.
I feel for you, mouth ulcers are the worst. There's a new pill I found that has really helped me ( The Canker Spanker ), you may want to check them out. TheCankerSpanker.com