Laura, i don't think it's too late at all. Many women and men change careers at forty - there is no reason you can't start one. I'm sure many employers down the track would probably look upon you with high regard - having the maturity of an older adult, and the youthfulness of only just hvaing studied - so not yet bored, etc.
My mum is 40 (I'm 21) and she has a 16yr old, 11 yr old & twin 11 month olds.
She completed uni as a single mum with the three of us about eight years ago, and now she owns two successful businesses. She is up to her neck with stress but it's her passion - don't let anything get in your way!!

Good luck!