A third vote from me for Rosemary- it’s a great universal herb in spellwork! 
The first thing that came to mind was ginger, but that works more on the fiery/yang/active energy side of things. For improving emotional state, I like Siofra’s suggestion of lemon balm- that is a happy and positive plant that should work great for emotional stability! 
I’d also put catnip in the running too- it’s a great one to soothe difficult emotions and help encourage peace 
Good luck with your jar spell, Assyla! 