It is simply not possible to end a romantic relationship without any kind of pain. In fact, it's unrealistic to expect to do so. The nature of attachment is such that even if a relationship isn't right, we can still become close and have to go through grief when it's over. Pain is an inevitable part of being alive. Our lives are filled with the comings and goings of people, events, opportunities, and so on.
Disappointments abound, setbacks are a part of the process.
When we have loss or disappointment, it's natural to feel a certain amount of pain, and unrealistic to expect to never hurt, especially with regard to romantic relationships. Suffering is what occurs when we seek to avoid pain by doing that which is ultimately not in our own or other's best interest. We numb ourselves with television, food, alcohol, drugs, and sex. We avoid making difficult decisions, putting off the immediate pain in preference of suffering over the long haul. In love relationships, we put off the break-up, naively expecting it to hurt less at some later point in time.