I discovered my wife and I had been using the same toothbrush for 3 months
How did you figure out that you had been using the same toothbrush?
I found out when I went to have my teeth cleaned at the dentist office.
It sounds like it was an accident, but sharing a toothbrush is not a good health practice as you have found out. Try using different color toothbrushes in the future so that you don't accidentally use the same one again.
I feel so embarrassed everyone has been talking about it to me.
You shouldn't feel embarrassed I'm sure that it was a common mistake. It's is preferred that everyone have their own toothbrush, so getting one that is not the same as your wife's is probably a good idea.