My dd's 9 months old and probably considered to be an 'easy' baby. I always knew I wanted children, even if she was say, 10 years too early lol. I remember not long after having her thinking 'is this it?' The whole 'I love my kid, but don't like motherhood' rings very true for me. People are usually quite shocked by that, or have been in my experience.
It's draining, repetitive and boring to say the least at times. And like the OP, I have weeks where everything is sunshine and buttercups, and then it'll all go to s**t. The screaming and shouting at you for whatever it is they need, can make you feel like you're being pounded with a mallet with every sound.
For me, yes I've found motherhood to be a huge dissapointment. I wouldn't change it or my dd for anything as she is so precious to me, but sometimes I do want to cash it all in for a day or two and just get back to basics, when it was just me.
I struggle and question myself all the time, but you only have to look at dd to know she's well looked after, loved and thriving. She is only 9mo and sometimes I do have to wonder how I'll cope in a few years time. Nervous breakdown? - probably!
Every mother doubts and questions, some just aren't brave enough to admit it. What you're feeling sounds very normal to me, I blame all the child rearing books and 'practical' parenting books you can buy when your preggers etc. They all depict happiness and how great it all is, they just left out the other half of the less happy truth which is - it's feckin tough and sometimes, you'll want to lob yourself at a brick wall to drown out the 'MUM!'