Breathe, Mom of one! I am a mom of one too, so I understand your dilemma, very much so in fact! Different beginnings, we had both graduated from college a year before getting married, wanted to wait to have kids, which we did, almost 5 years. I am an only and husband has two sisters, but they are 9 and 10 years younger! Hubby has a big, close knit family-lots of pressure for lots of kids and he is the only one in his generation who is settled and has a family, so even more pressure! We agreed we wouldn't want to more than two, but both kind of like the idea of one, some days more than others! Finances are different for us, we owned a home in another state, economy went south and we tried to stay ahead of the tide, by hubby accepting another, better, more secure job back home in Texas, sold house at a loss, have only rented since, carrying college debt around still, etc. Plus I gave up working outside the home to be home full time. So that's a factor for us.
Our only son is 27 months, a ball of energy, had a sick filled pregnancy, no problems, but a rough labor and delivery, and he was a colicky baby and still isn't a good sleeper. Plus, since his birth, I have struggled with weight and depression and I refuse to try and get pregnant under current circumstances because it's dangerous to my health.
We too are undecided, were more certain we wanted two before number 1 arrived. I go back and forth everyday and we always said we wanted 4-6 years between, so not even like we're running late at this point, just always thinking about it. Best thing I can say is, you're young, you have time....I am about 8 years older than you from what I can deduce, so age is more of an issue in my decision making. Meditate on it, pray about it, you have to find some peace before you make this decision.
I highly recommend the book, The Fertile Female. You may think the subject matter doesn't relate (it's about infertility), but when I was trying to get pregnant I read this and it really helped me clear my mind of all the pressures associated with having or not having a baby, being a mother, carrying a child for 9 months, etc. I did a lot of visualization of how I saw my life with a baby in it and what I needed....it's a great book!
You'll know from your head to your toes, in your gut, in the depths of your soul if it's right to have another and maybe right now isn't the time to know. You should communicate with DH on it too, it's important that you are both in the same place...very important!
Just know there are others out here struggling with the decision too. It changes everyday and seems like you have to know right now, but quiet your mind and heart and the clear answer will come on whether or not your family is complete.