Firstly, you are not a crap mum. I am willing to bet that the mum you met has days just like you do. It's easy to look in at another persons life, and see it as the perfect life. Believe me, i've been there. But we all have days like you have, especially when you have a baby and a toddler. My early days with 2 were horrible. I saw my life as an existence. My H got really poorly too (he had pneumonia), so I had a tough time.
Can you go to any SureStart groups? I used to love a baby massage class I used to attend. We were all much more open and truthful there!
I used to do a lot of arts and crafts with L when A was little. I'd stick A in the sling, or bouncy chair, and we'd do painting, playdoh, drawing etc. It meant that L got some attention from me that she so desperately craved. We also used to do a bit of cooking. She used to help to spoon the ingredients into the bowl, and stirring. She was only just 2 when A was born, but she loved to help and be a big girl.
But I look at my two playing now, and it's a great feeling, although dragging them both out of the Wacky Warehouse today whilst both screaming like some banshee children was not so great!