It's great you have found an outlet here for your feelings, thank you for the refreshing honesty.
You didn't make a mistake, unplanned pregnancy isn't a 'mistake' , its just something that happened, it's simply unplanned, so don't see it , or yourself in such a negative light!
You also didn't 'fail' your mum. Failing means to do wrong or hurt someone intentionally and you did neither of those things so you didn't 'fail'.
All this negative use of language runs off and affects the way we think about ourselves so change the way you think about the events and you may help think about yourself in a more positive way which can only be good for your state of mind.
Best wishes... :-)