I was told that by following our conscience, we each must decide for ourselves whether or not an act is sinful for us. So if I undergo sterilization with the intention of saving my marriage or because I can’t afford another child then sterilization could

What you were told is wrong. Some acts are intrinsically evil and cannot be done, even in order to secure a good, such as saving a marriage or living within one’s means. Scripture is explicit on this point (Rom 3:8). Only if an act is intrinsically permissible does the question of whether the circumstances warrant that action become relevant.

The proper procedure to follow is to first look to the Church and the sources of revelation to determine whether the act is ever permissible and, only if it is, then ask whether the circumstances warrant it in this case. One cannot pre-empt the former question by assuming that every action is potentially permitted.

Forming a true conscience and then following it is essential if we are to live morally upright lives. Sterilization done in order to prevent childbirth is never permissible, and so no particular circumstances including your own warrant it.