Thanks all for your honest responses.
If I could 'snap out of' how I feel, believe me, I would have, long ago - I honestly don't choose to feel like this. I do have a card ready to send and I will send it tomorrow. I won't let this sour my relationships with my family; my biggest fear is that my failings, feelings or reactions will harm others, so I strive, always, for that not to happen. But it's like draining the sea with a sieve sometimes.
I do count my blessings, far more than I used to: I can now appreciate what I have and I am glad of it and of simple pleasures. I have a roof over my head, food to eat and a lovely little family who are the world to me. I don't live in a war-torn nation. In all those things and more, I know I am lucky.
However, some things get to the core of who I am, which is inadequate and not good enough. I remember being 'graded' as a child by my mother, who occasionally set us intelligence tests: I always scored the lowest and was teased for it by my higher-scoring brothers. Just one example, and I can't blame everything on that - there comes a time when you have to take responsibility for yourself and accept who you are - but the attitude I grew up with has helped to shape who I am now, as happens with everybody to an extent.
Irrational though I know it is, I feel it for myself as well as my daughters when I hear of the (particularly academic) success of other family members, though I KNOW it isn't true (of them, at any rate!). I think I will always have these feelings, so I need to find ways of sitting with them and accepting them, but I find it unspeakably difficult and painful to accept that this is who I am, without despising myself completely, when I value kindness and compassion to others so highly.
LOVE the WTP quote, Stansgran!