((((((((((((((((((((cantstopcrying))))))))))))))))))))))
I agree with gimmeice.
i can understand why it is stressful for you.
If they are giving your responsibilities to this other person, are they giving you new responsibilities? Is there a new task that you could ask for, that you think might be a challenge for you?
Your boss might think it is a good show of initiative if you talked to him about it, saying now that you have these tasks off your plate, you would be happy to help him with x, y, z.

You AREN'T defective. You are a valuable important person.
Please be kind to yourself.
sending and happy thoughts,
ktgirl