Hey, I think you're being a little hard on yourself. I see you made mistakes negotiating your V in Berlin, but your mistakes seem more out of ignorance and NRE (new relationship energy), than out of being a "total shit" or however you are castigating yourself now.
Being out on a V date, as a hinge, especially for the first time, can be a delicate balancing act. All 3 can feel awkward, even if the arms are cordial, even if the hinge is trying like heck to keep things balanced. Your primary may feel like she is due more attention since she's been with you longer. The new partner may feel like she's due more attention because she's new and probably thinks she has more reason to be nervous and jealous than your longtime love does. ( ie: The first time I went on a V date- went out dancing- with my 2 current partners, my new bf felt he didn't get enough attention, even though I really tried to be 50-50 at the venue, and afterwards, shagged him and slept with him in the guest room while my gf spent the night in our bed alone.)
And here you go, not just a one night date, but a whole week, in a strange city, and making your primary live and sleep in your new lover's apartment! Her turf!
Yes, your communication levels sound like they sucked that trip, but you learned from it, and you sound like you're improving! Not trusting yourself now sounds like a rational response to me. It brings self awareness, a kind of psychic self checking, which is good, going forward.
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
Mags (poly, F, 61) loving miss pixi (poly, F, 39) since January 2009, living together since 2013
"Master," (mono, 34), miss pixi's Dom for 3 years