Welcome, Manarqest!

It's okay to be worried.

Your girlfriend domme has told you what she wants and you have voiced your concerns.

If "not being shared" is a hard limit for you, then let your partner know.

What works for the two of you, may or may not work for the three of you. Be up front about your concerns, be specific and be prepared to listen to her responses.

What do you want the relationship to look like in the future?
What does she want it to look like?
Are the two anywhere close?

If not, then you may have to assert yourself.

To your success,

Ruby