Every relationship begins with mutual appreciation of the other's efforts and a desire to make our partner happy with every possible tool. However, these feelings usually fade in time. As long as it fades equally, it does not lead to conflicts, but if one of you starts to value the other less than how much he or she is valued, then asymmetry will develop in the relationship.
One of you will do everything to make the relationship work, while the other manipulates him or her to do even more for the sake of love. This very manipulation is responsible for the deepening break in the initial bond. The easiest solution to this problem is to realize this imbalance and talk about it, but if it is too late, then both of you should consider ending the relationship. The person who is less valued would deserve a partner who appreciates his or her efforts, and not to feel lonely in a relationship.
However, manipulative behavior tends to create a sense of dependence in the less valued person. He or she might not be able to see that ending the relationship might change his or her life in a positive way, and they might want to hold on to their partner. In this case, they can only learn to appreciate themselves, put their interests ahead of the other sometimes, and learn to say no when needed. It is the only way to end the vicious cycle before not only your partner will not see your values, but neither will you anymore.