Can obsessing over a nightmare be part of a nervous breakdown?

Not sure: "nervous breakdown" can mean all kinds of things, and i don't know what you're specifically experiencing. Recurrent nightmares can indicate that your unconscious is working on these topics in some way. There really can be help from psychotherapy both in understanding such things and resolving them. "obsessing" can be treated also, but comprehensive evaluation is needed first.

Nightmares: The brain does not 'break down' but research shows that it is not the content of a dream or the frequency of dreams but rather how we react that is important. Please check on my book No More Nightmares-How to Use Planned Dream Intervention to End Nightmares. You can easily teach your sleeping brain that there is no such thing as a bad dream and you start sleeping peacefully through your dreams!