I love the statement in one of my iphone "baby" apps... "Nobody can ever truly afford children"...

Try not to worry too much, but know that you probably will have a good month of stress ahead of you. Tears and mixed emotions. I always tossed up on having a third, when was a good time etc etc. I wanted my family finished before my 28th birthday (didn't see the point in having children over 3 decades like my Mum). When I fell pregnant with my third which was also very quickly, I was excited. I was scared, I was depressed, elated, anxious. A huge mixture of feelings. I'm 5 1/2 months pregnant now and am feeling great! I'm not looking at purchasing a house for about another 8 years and am focusing having these qualifications almost finished by the time bub arrives (well partly). I'm studying to be a Musculoskeletal therapist (Bach of health sience) with a diploma in massage therapy, to combine my knowlege and treatment styles. So this keeps me going.
So far in life I've found if you really want something, regardless of if you have children or not, you WILL make it happen. So as hard as it is try not to stress out too much and enjoy your pregnancy Go and see a counseller and have a good chat, there are pregnany counsellers, which some of them can be wonderful!

You really will be fine :-) And congratulations on the BFP