My fiance and I are getting married this August, just like you. For us, it will be right before our final semester as undergrads–I’ll be 22, he’ll be 21.
I think it’s important for every engaged couple to really discuss their views on using NFP when they get married, and to make sure they are on the same page. It takes a lot of prayer and a lot of discernment–along with a lot of conversations between the two of you!–so it’s good that you’re both aware of this.
The guideline from the Church is that a couple must have ‘just’ or ‘serious’ reasons to postpone/space their children. This is important to keep in mind, but it has to be interpreted by each couple. A reason which is serious for one person may not be serious for another, etc.
I really understand where you are coming from. It’s scary to think about having a baby when you don’t think you’re ‘ready,’ when the time isn’t ‘right,’ etc. But like Mirror Mirror said above–we are called by Jesus (and JP2!) to “Be not afraid!”.
Does this mean you should throw all planning out the window and rely totally on providence, sans NFP? Nope, not if that’s not what God is calling you to.
If you sincerely believe that you have discerned that God is calling you to a certain vocation that requires considerable graduate work or career establishment before kids, then I think you *may *have just reasons to avoid pregnancy. But I’m not here to tell you that. Pray about it, talk to your fiance, talk to a priest.
What I can tell you is that I am discerning graduate studies to begin around the first year of our marriage. Do I know if I’ll start–or if I start, whether I’ll finish? No. We have talked about it at length, and even though I’m feeling the baby fever (hehe, who am I kidding, I’ve been feeling it for the last year!

) we know it is best for us to postpone pregnancy *at least *until after we graduate in December. Our reason for waiting is the schoolwork and pressure I will be under (stress is not good for pregnancy!) and the fact that we won’t be in the most stable situation. If it happens, it happens–and the baby won’t be born until after we graduate anyway. We believe that it is most responsible for us to give us that time to get completely settled in our life together before bringing another life into the picture.
But God has a plan and I trust it. Life is an adventure and we really aren’t totally in the driver’s seat. We are called to discern God’s will for us, and trust that he will take care of us. Take it one day at a time. Everyone has their own experience and their own take on it–only you and your fiance can know what God’s will is for your marriage.