Wow Brad, it's great that you are concerned. Because phen does cause mood changes. Did she get the phen online or from your doc? Maybe talk to the doc about this, as it could get serious.
My sil was fired from her job cuz the phen was making her cranky and she was being rude with the patients, and who wants a crabby nurse?
Anyhow, maybe talk to other family members too, maybe do a mini intervention or at the very least, let her know some of the things that are happening to make you feel this way.
She can't take the phen forever, so she should have good eating habits down by now.
Good luck and God Speed Brad. Pray it will work out for you and wifey. 18 years is a long time, and it sounds like you have a happy, beautiful relationship.
Missus