You may be lactose intolerant and that might explain the milk and stomach problems. As for the pepsi, not sure as above could be the caffeine, but it could also be the high sugar content. If it's causing you problems there is no sense drinking it.
As for "don't tell me to stop because I can't" don't be ridiculous, you can stop drinking those things you are choosing to continue to drink them. What is it exactly you want us to tell you? That you should totally continue drinking them even if they cause problems? That would be silly.