I'm dwelling on the fact that my former roommate kept some of my dishware without asking
What was your reason for getting upset about something like this?
I feel so angry about the fact that she purposefully left a knife behind. One day I found a bug in my room, and it's a known fact that bugs breed in plastic.
While it is not good to take someone else's belongings, it is also not good to hold a grudge. You should give this person a call and explain to them how you are feeling so you can get it off your chest. This will at the very least allow you to feel better.
What does it mattter as long as I'm feeling better?
Well if you continue to hold in this feeling than it will be difficult on your relationship with your roommate. It is also good to get into the habit of not holding things in so you are not walking around with additional stress.