thelady, I'm completely rubbish at expressing, but managed to bf my 2 long term (after a very, very hard start with dd1). So I think it's important not to draw too many conclusions about the expressing.
Maybe you're just a fellow member of the Useless Expressers Club - we get fewer nights out than those dairy cows who can express pints but otherwise it doesn't matter much.
You know, even a little bit of breastmilk has loads of goodies in it, so even if you were to stop right now, your dd has already benefited from your hard work. Do you think it would help to focus on the benefits that are 'in the bag', rather than worrying about possible missed benefit in the future? I think that's what I'd aim to do in your shoes.
With the latching: I had terrible problems getting this right with dd1. The worst times were when she was hungry - she would panic and flail about and it was horrible. Sometimes I would just let her latch on any old how (toe curlingly painful), drink a bit and calm down, then try doing it properly.
You might like to try a less painful version of this! ie start her off with a bottle so she has something in her tummy, then latch her on.
I'm not a bf expert so am giving this advice a bit cautiously, but it worked for me.
Wishing you all the best.
Aargh, just read thread and I see Aitch has already made this suggestion! Oh well, will send anyway!