To be blunt,
I think you need to start over. A cover letter isn't about you or what you want, or your career goals. A cover letter is about your employer.
This is what a good cover letter looks like: What does a good cover letter look like? â€" Ask a Manager
Do not talk about how this position would benefit you, i.e. , "This position would enable me to continue learning about the specialty that has been my passion since before nursing school ever began."
Instead, talk about how YOU would benefit the employer - this SPECIFIC employer. Your cover letter could be applied to pretty much any OB position at any hospital, and is therefore too generic. I think you need to mention specifically what you can do for this specific hospital. This would be a good time to throw in some of the research that you have been doing on this hospital (you have, haven't you?) for example, if they're a level III perinatal center, or if they're magnet status, or they recently hosted an OB symposium or conference or something. Bring it up in your cover letter because it shows that not only do you want to work in OB, but that you want to work in OB at this specific hospital. This will impress recruiters and hiring managers more.
I would not bring up clinicals so generically, as that is basically reiterating your résumé. Instead, this is a perfect opportunity to tell a BRIEF story. For instance, you could say, "my mother-baby nursing instructor commended me for bla bla bla when bla bla and this illustrates how I would excel in a role as a care giver at whatever unit at bla bla hospital." This is the kind of stuff that is not on your résumé that can really make you stand out.
Don't throw too much in though, three paragraphs are ideal.
Not trying to be too harsh. I can sense that you really want this job. Your competition is probably sending in terrible, boring cover letters. The way to get this job is to make your cover letter stand out from the usual boring ,"I think you should hire me because this is my dreeeeeaaaam" cover letters.