Writing a song with someone else means losing some lines you loved. The song usually wins.

Collaboration has a real cost, and the cost is specific: lines you were proud of will not survive. That hurts, because you can hear the version where they stayed. What usually happens, though, is that the finished thing is better than either of your solo versions would have been, since two people catch different weaknesses. You trade your favourite parts for a stronger whole.

Keep a file for the lines that get cut and put them there instead of arguing for them. Nothing is lost, only postponed. Then judge the session by whether the song improved rather than by how much of yours stayed in.