"...always loved the idea of having a couple of kids, fab husband, glittering career and plenty of time on my hands"
How many have you scored so far ?
Seriously, we all have dreams when we're young, and we all end up with compromises. No one else can make up your mind for you, especially your mother sp beware of her desire for grandchildren.
And you certainly don't come across as a cold-hearted b*tch.
As regarding future regrets about remaining childless, say you had several children now, you might end up regretting that in the future. We all have to live with our life's choices, and while the grass seems greener on the other side, it usually isn't.
Happy Birthday for a fortnight's time. (how I envy you your youth - I'm over twice your age LOL)