I have a real dilemma about whether or not I should tell my manager about my mental health problems.
It is something I really want to avoid as I don't want anybody to know (besides my GP, only my sister knows and that's because she lives with me), and I don't want to give him any reasons to have a lower opinion of me.
But I feel like I am being dishonest in not telling him, especially where aspects are affecting my work.

The main problems I have are that I have started having CBT one morning a week and going straight into work from there, but I find the sessions are really hard work and I feel drained afterwards so I know my work isn't as good as it should be on those days.
Also, it came up in my performance review that I need to ask for help more, and to speak up when I need things. This is something I find incredibly hard to do. I am working on it, but it will take a long time and I don't want him to think I am just not bothering.

I work for a small company and we have no HR department, so if I told anyone it would have to be my manager. He is a nice guy and, so far, has been really supportive - he knows I suffer from bad headaches and migraines (though not that these are due to anxiety/depression) and has said that I can go home any time I need to, or just take a long break in the staff room.

I suspect that he knows something is up anyway, because I had to ask him if I could come in late one day a week (for CBT, though I didn't tell him why) and he asked if there was anything he ought to know, and if I was ok.

So, I am really torn, I feel like I should say something because I hate feeling like I am lying to him, but at the same time I am really scared that it will negatively affect his opinion of me