Answer: "Over the years" you've apparently conducted an extensive trial of an "amiable way" to differ from your boyfriend, a trial that prompts me to snark, how's that working for you?
The way to say no is with the conviction that you have every right to say no. Period. The best peacekeepers aren't the softest ones at the table; they're the ones who are open to other viewpoints while letting others know there are lines they cannot cross.
It's helpful when the person you're saying no to has a 100 percent chance of surviving the disappointment of not going out to dinner that night. Helpful — not necessary.
So, stop treating your boyfriend's good mood as your goal, and replace it with the goal of being true to yourself, be it over something as minor as grocery shopping or as major as the way you handle money or raise children. Reflecting your true needs, wants, and motives — even in coming to a compromise — is the only way to live honestly. That, in turn, is the only way to get a relationship to work without bleeding one of you dry.
What he needs, meanwhile, is for people to air their resentment of him before it's too late for him to make amends. I've filled columns on the ills of the silent treatment, but I'll condense here: It invalidates you. Tolerate it, and you invalidate yourself.
So, give him a chance to see and fix the problem. For example: "I'd like to be able to go buy milk without getting the silent treatment. Which would you rather have, my going along just to keep you from punishing me, or my occasionally doing something different from what you have in mind?" Then, greet any future tantrums as follows: (1) "Suit yourself." (2) Do nothing about the ensuing tension. Go about your business and let him find you if he wants to make nice.
Silence and pouting could be what he grew up with, and learning healthier ways could take a while, even with sure-handed companions who know how to be kind, judicious, and firm in holding the line with him.
If you, meanwhile, were raised to seek conciliation at all costs, then breaking your self-defeating emotional habits might be too big a job for you to be the one who helps him break his.
You probably stand a better chance with the guidance of a good therapist, but please know that if your boyfriend doesn't see the problem with his huffing and puffing — if he shows zero willingness to hold himself accountable — then please see that there's no peace here to be kept. That is, not unless you break up.
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