Miaowmix and spunbodge completely unnecessarily nasty and against talk guidelines posts have reported both of you.
Fluffy you need to tell your mum ASAP really she needs to know so she can support you both.
As she's been pregnant herself at a young age she'll know "shit happens" so yea she might go a bit bonkers at first but she will calm down and help you.
You can't do this alone, you need the emotional and practical support.
Every first time mum is scared. And not just about childbirth!
I was nearly 30 when I had my daughter and I'd been a nanny for several years before and I still felt clueless! 
On the plus side I didn't find Labour especially painful not everyone does. But don't be reluctant to ask for pain relief if you want/need it. That's what it's there for.
My mum shelled babies out like podding peas but she still used gas and air.
You'll be fine, and you haven't ruined your life, I've friends who've had babies at all ages, youngest was 15 to look at their lives, careers, relationships etc now you'd have no idea of there being any difference (the 15 year old mum is now a barrister!)
It all works itself out eventually.