It's normal to be scared -- this is a big, scary decision! I totally understand your fears. I'll give you my weird perspective; this probably really puts me in the minority.
From the time I was 5 years old, I absolutely did not want kids. This feeling grew and grew as I got into my teens and twenties. I found most children irritating and boring. I simply couldn't understand when people described a fat, drooling, ugly little creature as "cute". I met my future husband when I was 20. When we started to talk about a future together (when I was around 21), he brought up the subject of kids, and through many tears, I told him I didn't think I ever wanted kids. I was sure he was going to call the whole thing off at that point, but he thought about it and decided he could live with it. So we got married. Fast forward several years -- both of us in our late thirties. He told me the child issue had gotten really big for him. In fact, it was tearing him apart watching his siblings and friends all having children, and believing that was probably not in his future. That got me thinking. At the age of 39, I had this sudden surge of realization that just about knocked me over: that if I didn't try to have a child, I would come to regret it for the rest of my life. My husband (who is a really wonderful guy, and who has taught me many things through our almost 21 years together) and I talked everything through, and finally decided to go for it.
So, here I am, at the ripe old age of 40 (41 in 5 1/2 weeks), pregnant with my first child! I was terrified at the beginning when I first realized I might actually be pregnant. I had myself convinced that it would take at least 6 months, maybe a year, because of my age. It actually took just 4 months. I hated putting on weight, hated losing my nice waistline, hated my lack of energy, and was really scared of what would happen to my life and my body. I had a lot of the same fears you have. I was really ambivalent about the whole thing until my first ob/gyn appointment in February, where I heard the baby's heartbeat for the first time -- then I fell in love! The following week, when I had my first ultrasound, it was even more exciting. The biggest highlight for me was when I got my amnio results back (I did all the tests because of my age) and found out I'm having a girl. Now I am really excited and looking forward to my new life. Yes, it will be scary having a reduced income for awhile, and I'm not sure how I will handle the new responsibilities. But I'm confident it will work out. I'm now 28 weeks along -- due date is August 18.
At this point, I am very happy and don't think I will ever regret this decision. It may be too early to say though -- I have not yet lived through the sleepless nights, worrying, having to give up my freedom, etc.
I have not modified my fitness routine very much at all. I've been a part-time aerobics instructor for 18 years -- I gave that up two weeks ago because I didn't feel I wanted to make that sort of commitment past the start of my third trimester. However, I'm still Spinning 3 X per week (somewhat reduced intensity), doing Cathe and Firm tapes (have not reduced my step height, but I have had to reduce my weights -- I'm using 10 and 12 lb dumbbells where I used to use 15 and 20), and doing Yoga. I work out just about every day. I still feel that I am in very good shape -- even though I have put on over 30 lbs, and am starting to get the occasional shortness of breath when I climb stairs, I feel basically good and pretty energetic. My husband knows how important my workouts are to me -- he is quite athletic as well -- so we are planning to make sure that we each have enough time after the baby is born (and after we get past that first sleepless part) to pursue our fitness goals and other interests. It will probably be a challenge to make sure each person has enough "on own" time, and we have our togetherness time, and lots of time with the new kiddie as well.
My oh my, I do go on, don't I ... sorry to ramble. I guess I'm just trying to say that it's perfectly understandable for you to be scared, and it really is a big and important decision, and you will need to make compromises. I am someone who wrestled with all these things and am very glad that I made the decision I did. All the best to you!