QE - Sorry to hear you're feeling down, I have been there and I know how sad it can make you.
Like others have said, you're only 18 and got alot of time ahead of you ^-^ I know being round babies makes you mega broody and makes you want them. But I think it's also important to live your life a little first :3
I've wanted kids since I was 23 and it was horrible having to wait (plus I was in a relationship where my ex didn't want kids so that made it worse) Got with my new partner and finally fell pregnant at 26 when we were financially stable. I love my little one to bits and wouldn't change it for the world, but I think it's important to live some of your life first

I had holidays, attended many expos and had alot of me time before I had my little one and I'm pleased I had that time and waited till I was ready.
Mid 20s IMO is the perfect time to have a baby as it reduces the risks of miscarriage and that way you've had plenty of 'you' time before settling down with a life long commitment. ^^
If you ever wanna chat though, feel free to PM me :3 Sorry for the essay xD
I feel exhausted.. but... coffee is helping :B