Should I tell my boss

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

No. I would not volunteer this information.

I agree, don't tell. Work with your T on how to ask for help. If anything, I would only explain that you are in treatment for some health issues, so that it's some kind of record for protection, but I would be vague and refuse to give any specifics.

Seesaw

Do not say anything that they can use against you. You can say you are in treatment for a health issue and they are not entitled to know anything further.

Let me add to the chorus of NO, don't tell and do whatever you can to move your CBT to after work (even if you have to change therapists). Therapy is draining and it's best done after work, not before.

Thanks all for confirming what I thought. I don't think there's any chance of moving CBT. I am having it through the NHS and the psychologists only work business hours. I might be able to ask for an end of day appointment but would probably go back onto a waiting list till there is a slot available at that time (and I had been waiting 8 months to get this slot)