First, be sure to remain calm--not easy in your situation. Mediation works if people participate productively and since nothing compels one to remain in mediation, insulting or attacking the other party would likely cause them to leave. So, you should think like a lawyer--make a list of your points/arguments. Have back up. If you've gotten a job, bring proof. If you live in a pleasant environment bring pictures. If you've gotten training, bring certificates of completion. The more you calmly demonstrated your suitability for what you seek, the more the mediator will be able to help you. Remember mediators look to the parties to arm them with arguments, documents, proof and anything which will help.
Finally, if you compliment your spouse on the good things she's doing (assuming it's true) she might soften her resistance.
First thing to lessen the nerves is to learn about the process. See Nebraska's official court mediation site for information:
Then go over in your mind (and it may help to speak it out with someone else) what your goals for the mediation are - not just custody, but how many days, where, etc. Also plan out beforehand what you would be able to be flexible on - for example alternating holidays vs. always having the child at Christmas, etc.
Also be aware that Nebraska has a Parenting Act. Be sure to understand it. If you do not see a family-law attorney qualified in your state.
Disclaimer: This answer does not constitute legal advice. It is recommended that you consult with an attorney liscensed in your state and with a practice focused on your issue who can review all the relevant facts and issues of your case. This communication does not create an attorney-client relationship nor the duties associated with that relationship
Sign up to receive a 3-part series of useful information and advice about child custody law.