Hi Camille, 

I notice that sometimes after a practice session of high notes and mixed voice exercises, it actually becomes harder for me to sing some songs with higher notes and in mixed voice right after. I make a really conscious effort to have good technique, but I notice that I tend to push for higher notes in mix voice. Is this just a result of vocal fatigue after a session? Thanks!

That could be, Marley! How long is a typical practice session? I'd recommend warming up for 5-15 minutes (including mixed voice) and then getting into your song work.