You should not have to wait for 3 months to get feedback and an evaluation. You are going to have to be assertive and ask for the feedback. Ask, "how do you think I handled that?", "is there some way I could have done it differently?", "is there something I should be doing that I'm not?", "what do you do in a situation like that?". The fact of the matter is that most nurses who end up being "preceptors" do it because it is required of their job. They are not trained instructors like you had at nursing school, so many times they are not always going to be sensitive to your needs as a new grad. So, the burden is on you to seek their advice. Therefore, ask, ask, ask. "How did I do?" And, if you can take the criticism, because some people can be incredibly blunt, ask for it, but be prepared in case it's not going to be what you want to hear. Ask, "what should I have done instead?" You are going to find that each nurse does things, and feels about things, a little differently than another. This is where the nursing theory you learned in school helps you to decide and sort out advice that is worth taking or not.
It is not that the field isn't for you, it's just that you are feeling kind of lost in the shuffle right now and feeling a bit depressed about it. It takes many, many months before you begin to feel really confident in what you are doing. That will be anywhere from 6 months to a year. Everytime you have a patient who experiences some particular complication of labor or delivery focus on reading up on that when you get home. Pick one subject at a time so you don't overwhelm yourself. Make your experiences an extension of what you were doing before in your clinicals at nursing school. You only touched the surface of L&D in nursing school. Now, you are fine tuning that along with having to learn how to be an employee as well. It's a big task. The difference is that you have to do this analysis and learning on your own now. So, hang in there. It's not over until you are told it's over. You stick with it until someone tells you it's time to leave. And, then, ask that they allow you to resign rather than be fired, or be assigned to another unit. Don't make any decision to leave this job or you will forever wonder how you would have faired at it if you hadn't quit. Chances are that you are doing what is expected of a new grad, but since you've never been in this situation before you really don't have the experience to evaluate yourself in this.