It seems like you are a generous person to think of your girlfriend, that she deserves better. If you love her and she loves you for the way you are ("warts and all") then I wouldn't be so swift in letting her go IMHO. If you don't love her then let her go.

You are very lucky to have someone like that. If you let her go and she agrees then it may take a long time for you to find someone as good or better than her. In fact, you may never find someone as good as her again.

I think that you should tell her how you feel. Don't emphasize about the letting her go part. Just talk more about you thinking that she deserves better than you. I think that she's not going to agree with that and probably would not let you go easily.

I think that the joint counseling would be a good idea so that she would understand you better. But it seems like she understands you very well enough as it is. You are very lucky to have that because not many would take the time and be patient with someone with depression.