Wow. I can see how that must have been some curve ball!
I know it's not quite the same, but I did get accidentally pg when my first child was almost 14. At first I couldn't imagine starting over, and my husband didn't want to even consider it at all. Then I had a m/c and was actually pretty heartbroken. So a year a half later we're now expecting a baby (that we planned) and my son is 15 1/2. Sometimes I still wonder what we are doing...starting over when one child has only two years of high school left. And frankly, my older body has just fallen apart this pg. But I know there will be good things that come of it too. I love the new baby smell, and I love the cuddles and kisses. I love the really funny things that toddlers and preschoolers say. I love how my kids keep me active and young, and that Christmastime and other holidays just wouldn't be the same without a young child around.
I hope you that you find support from your family and friends. And just because you're no longer with the father, it doesn't mean he shouldn't be a part of the child's life and be contributing financially. But in the end, only you can decide what's best for you to do. I wish you luck!