I'm sorry to hear about your frustration. As far as searching for posts by a particular user, you should be able to do this by clicking 'Advanced Search' in the top navigational bar, then scrolling a bit and entering the person's username.
You mentioned 'im not notified like before when someone posts on my topic.' I'm surprised because we haven't made any changes to notifications that would remove your being emailed if someone comments on your thread. Can you give me a little more information on what you used to see, so I can try to figure out what's going on?