Medical school is a very challenging and exhausting experience. Residency is even harder on relationships than med school is because of its very, very long hours. Relationships very often take a back seat to the career during med school & residency.
Both daughters had relationships implode due to med school and the stresses imposed on them by med school, but both also found life partners during med school whom they are still married to/living with long term. (I will note that neither of their partners are physicians nor aspire to be physicians. One is a professional in a field wholly unrelated to medicine; the other is working a day job while he prepares to return to school .)
The two elements in both successful long term relationships:
a) both partners in the relationship share mutual interests and activities that have absolutely nothing to do with school and work.
b) each individual in the relationships is autonomous and has a pretty high level of emotional independence. IOW, neither partner depends on the other to fulfill their entire emotional needs.
Each individual has their own friends/hobbies/activities separate from their partner’s and they all have friends separate from “couple friends”. Each individual is Ok with engaging in leisure & recreational activities without their partner. Flexibility and a willingness to compromise is extremely important. Also a willingness to share chores (or at least agree to share the cost of hiring those chores done) is also key.