Awe, big hug! You are going to do fine. Self doubt is fine. I still remember clearly sitting in the car with DH on the way home from one of my last midwife appts with my second and saying "Birth hurts! I don't want to do it again." and him saying, ever so helpfully, "It's a little late for that now." Ha Ha...
Truthfully, esp the first time, it does hurt. Mostly I think cause it is all new, and it is unknown. BUT, you can help. Learning as much as you can about the process is one way. Learning what you can do to help deal with the pain of the contractions, things that help you feel like you are not just along for the ride.
From my first birth I took away a very vivid memory of what the contractions felt like at each phase. I'm probably using really old terminology, but here is my description of labor:
In early labor the contractions start at the bottom of your belly and move up. This is when you are dilating and effacing, the "thinning" of your cervix, it can take awhile, and initially feels pretty much about the same as mild menstral cramps, you know they are happening, but you can carry on with life mostly as usual. As this phase progresses they get stronger, and longer, and you get to the point of having to actually think about them, stopping and making an effort to relax.
Then comes 'transition'. In my opinion this is the part of labor that makes you want to leave and come back later. Which makes sense to me, because this is when you go from an upward movement of the contraction to a downward. The muscle(s?) of your uterus are shifting from the "thinning" to the "pushing". They feel like the muscle crashing into itself, the contractions started at both the top and bottom and "met" in the middle. It felt so unproductive. The blessing is that it usually doesn't last very long (the used to say about 20 min, but I'm not sure about that now) This is when you have to just imagine you are in the ocean and the contractions are waves. One at a time and only one. Rest and relax as best you can in-between, and do your best to not "ride the wave you are not on" be very "right now".
After that comes the pushing/delivery. To me the pain felt like it went away! I know it didn't, but suddenly I was a full on active partner in what was happening, and I think that gave me the mental boost I needed. The contractions start at the top of your belly and move down. The very best thing that I was told to do was to relax through a contraction, do nothing and just feel what my body was doing. Talk about power! Those contractions are A.MAZE.ING! With your back issues, I'm not sure how that would work for you because I remember my body jerking forward and back. With the next one I was able to work with it, to use it and direct my energy into it. The pain at this stage is so productive feeling, that it just doesn't seem like pain in the traditional sense. The biggest thing to be careful of at the end is being sure you are resting in between contractions so that you don't wear out.
Then out comes the baby and the world and the pain and everything else just goes away. I always tear, and I'm almost never aware of the stitching up :)
I hope this is helpful to you, I hope that you get over your nerves and have a birth that results in not only a healthy baby and mommy, but a birth that you are pleased with. You can do this.