If a husband and wife have multiple children, and one spouse feels emotionally and physically that a break from pregnancy is needed but one spouse will not abstain, what should be done?
I would counsel that the one spouse defer to the other spouse who feels emotionally and physically that a break from seeking to become pregnant is needed. As Bl. Paul VI wrote:
With regard to physical, economic, psychological and social conditions, responsible parenthood is exercised by those who prudently and generously decide to have more children, and by those who, for serious reasons and with due respect to moral precepts, decide not to have additional children for either a certain or an indefinite period of time (Humanae Vitae 10).
Taking such a conjugal break is how, for example, a husband can sacrificially lay down his life for his wife (see Eph. 5:25). St. Paul also specifically encourages couples to take needed breaks from conjugal intimacy (1 Cor. 7:5).
Perhaps you could get also the helpful advice of good married Catholic friends and/or your parish pastor. If necessary, consulting a good marriage counselor who is respectful of your Catholic faith could also be helpful.