I'm spinning in circles trying to do this.. I believe I would be looking over something.. not, any help could be greatly appreciated?

I'm focusing on a training website where I've 10 or even more sports athletes. I want an answer where ( me like a coach ) can individualize each athlete and upload different agendas/practicing all of my sports athletes??

So, maybe eventually I'm able to upload PDF training to Athlete 1, after which simultaneously, I'm able to upload another training to Athlete 2.. after which Athlete 1 can sign in and can only see his/her training with no one else's training..? So, it might be very individualized per Publish -- but exactly how to complete when a sports athlete logs in just sees their training.. ?

Well, you will need to begin with customers. Give a user for every of the corresponding sports athletes as logins.

Should you fancy really getting knee deep in code, write a wordpress plugin. Otherwise, look for a document management wordpress plugin that provides support for customers. Check a few of these. Such as the user access manager that might provide the same kind of functionality.