To say what you are, i.e. what profession you have, we say Dw i'n ...'

The pattern for 'I'm retired' is a little different; we say:

Dw i wedi ymddeol

Sometimes we change the order of the words around for the purpose of emphasis.

This sentence emphasises the fact that you're a teacher. i.e. It is a Teacher that I am.

You can't use this pattern for talking about places of work, or for saying that you're retired.