So this isn't about being "secondary" -- this is about "I want to be in an Open V now rather than in a Closed V."
Seems easiest to just tell them both:
"We are 6 months in. Initially I was good with a Closed V, but my needs have changed. I would like to be in an Open V that allows me to date others. Could you be willing to participate in that?"
They are either willing or not willing to participate... but that answer comes after the asking.
You ASKING is you doing behavior to help meet your own needs. You not asking
is you NOT doing behavior to help meet your own needs. Making your partner and meta aware of your changing needs is making them aware. It isn't a "confrontation." They cannot be mind readers.
"Emotional courage/confidence" does not come raining from the sky. It comes from exercising that skill. If you want to grow it stronger -- be willing to exercise it. It only grows by doing.