Healing a broken relationship will be a challenge. How much of a challenge will depend on many things. The first things you should go about fixing your relationship is why the relationship is in trouble in the first place. Is your relationship broken because of infidelity? If so, was it you or your partner that cheated? This type of relationship can be fixed but it is the hardest thing to overcome and both partners have to be willing to work very hard to make things right. With infidelity it would seem like the partner who cheated would have to do all the work, but that is not really the case. The truth of the matter is that it will take for the person who was cheated to try to overcome their fear of being hurt again, as well as their anger and desire for revenge.
If your relationship has broken down more gradually over time, this might be a little easier to manage. Of course, it will still take time to work and both will need to be 100% committed to working on it. Many relationships in this category die like a plant in a garden, from lack of nurturing. It is not usually a big thing that ends it but rather a series of small, seemingly unimportant, things that will weaken the relationship to the point where it will break very easily. This type of relationship will take an honest assessment of what each of you has done, or not done, to weaken the relationship.
Once you both admitted that you have played part in the breakdown of the relationship, it is time to sit down with your partner and honestly discuss what went wrong, what you think needs to be done to make it work, as well as what you personally are willing do to fix the relationship.This part of the process will be very difficult and will often lead to some unpleasant argument. Why? Because this is the part where you will have to listen to your partner about your partner's unhappiness with you. This would not be easy for you to hear. And the same goes for your partner. Very often one partner would not be able to deal with what they perceive as criticism when trying to explain the feeling of unhappiness. This is the point where you, and your partner, will need to sort out your differences rationally. If neither one of you is not able to calmly listen while trying to explain it would be very hard to work things out.
Healing a broken relationship is not impossible, but it will take time and effort to work things out. If you both are able to face your faults and be willing to accept your differences ,then you will have an easy time of fixing your relationship.