Do you have a browser setting such that it remembers your login credentials? Which browser are you using and have you tried different browsers?
My wife and I have the exact probelm. And it is new in the last week. Does comcast have a solution other than getting a different email account?
The old tried and true that everyone hates to hear: try clearing your cache and deleting cookies associated with Comcast/Xfinity in your browser. Post back if that doesn't work.