Tips:
Relax. Detach. It’s just code. You just finished it and you want to be done and be victorious and move to the next thing. That’s understandable. But this is a long game.
Never reject advice immediately. “So, I’m not sure I like this, yet. I’m probably missing something. Let me think it over and play with it tonight, and let’s discuss this again tomorrow.”
Assume good faith. Try to give them the benefit of the doubt. Can you explain back to them why they want the changes, in a way that they agree with? Try it. Even if you disagree, they will see that you have heard them and understand their perspective.
If you disagree with their perspective, you can ask them about it, and why they value this over that. Keep an open mind. Later, a good question is, “Do you think we need to worry about <counterpoint>?
If you are sure they are wrong, consider using priorities or authority to deflect. “I could make these changes. Do you think that I should do them before or after ___?” Make sure you know exactly what goes in the blank, i.e., what are your organization’s priorities anyway? Or, “I think I will need to talk to my manager before I spend more time on this, he really wants me to focus on ___.”
If it is dysfunctional, get help. Tell your manager what you think is happening and ask them for advice. Can they see a way to make this less stressful? If it is really bothering you, your manager should care a lot and should help you work resolve this. If they don’t, you have a crap manager and should make a change.