What should I do if I forget password or account to log in ?
You can reset the password via email or follow this:
https://bitcointalk.org/index.php?topic=497545.0
Until now I have not got a clear explanation why the number of activities stuck in a certain amount and sometimes decreased.. 
That's funny. You seem to understand it below. And your post count is going down because you're a shitposter posting in off topic spam threads.
Clearly, 14 of your total posts will be counted as 14 activities every 14 days, so the rest will remain the number of posts.