Take a deep breath and relax a moment. And just for added comfort, I am preg. right now with you, totally unplanned also. My husband and I had just moved from 1600 square feet house that we sold because we needed the money (to pay bills and for our 9 mos. old) to a 500 square feet hole in the wall and found out we too were preg. again, despite the precautions we took.
I've had a little more time to adjust than you and this is what I would suggest.
1) Remember that every child is a gift and this is a surprise, not a mistake!
2) Find a local organization that supports women in crisis or unexpected pregnacies. There is one near me (extreme north TX-Gainesville) that is called ABBA (All Babies Born Alive) that does medical visits, sonograms, preg. testing, gives vouchers, and donates baby gear, maternity wear and nutritional needs to moms and families who want the child born alive. It even has nurses and midwives on staff and offers a lot of referrals to family friendly drs in the area. It is not a religious organization, but is given aid by several local churches esp. the catholic churches (Catholics are anti-abortion), as well as individuals and other outside funding.
3) Don't stress about the size of your belly already. Like I said, I'm on my second preg. and you are on your second in 18 mos. Most women need AT LEAST a year for their womb to shrink back to it's previous size after preg. and for many women, it is more like 2 years+ or it will never be the same size. For that reason, you will show much earlier and be much larger with this one than before. I am a house compared to my first preg. I still have 12 weeks to go and am very close to the size I was 2 weeks prior to birth with my first child. Just try to eat healthy and don't worry about the rest. Your body knows when you need to eat!
4) Know that many, many people have had un-planned pregs. and many, many of these families turned out just fine. Yes, there is an initial shock, no lie, but remember that you are still fortunate to have what you have that some expectant mothers did not (a home, husband, food on the table, some income, etc) and you are not alone.
5) When you visit your dr, talk right away with the business office and set up some sort of financial service. Most baby drs. don't bill until the child is born except for any lab type work done in office. ALso, this being a medical bill (as opposed to credit cards, etc), as long as you pay something (and it is usually very minimal) EVERY MONTH, it can not be counted against you and you won't be charged any interest. Often, if you talk to the office and tell them you will pay cash, they will give you a significant discount. Just get all of your options up front and make sure they take your insurance. Every dr. I have ever had (for any med. reason) has worked well with this method.
Best wishes and remeber it will all be ok. You get through it no matter what. You never know how strong you are until you are tested. Then, once it is over, you wonder why you stressed so much.
Hope I helped!