Disbelief may occupy some space in any relationship, over a period of time, based on a given sequence of events between the partners. Most of the times the partners end up fighting out the misunderstanding than the truth. Whereas the matter needs full attention and work out, it also helps if the partners open up and have a healthy confrontation. Silence between the partners might kill the show beyond anyone's imagination; it only adds up the distance if not anything else.
With whatever the understanding I have on this topic, I would like to suggest the below.
Disbelief is not a mistake of your partner. Feel the responsibility yourself for any disbelief shown by your partner. Try and understand where it would have probably gone wrong rather than pinching him or her for expressing the disbelief. Feel good about the openness in your relationship than pointing finger at the other person for being doubtful. Try your best in making your relationship come out of the expressed disbelief. Remember that it can happen to you too! May be at some or the other point of time, you may end up in disbelief too. Do not blame. Blame game does not work in any relationship, it turns out to be miserable if it happens between the partners for life long! As much as possible, don't feel bad about the other person for being so mean and all. Don't let that thought come to you. Understand that it's quite human and move forward in right spirit. Contribute your maximum in building back the trust. I don't believe in saying that if the trust is broken once it's broken forever. It can be worked out still. As long as the partners wish to be in the relationship, every thing is possible, every miracle must be hopped for.
If you are the one expressing disbelief, first of all don't feel bad to have been doubtful about your partner. Being doubtful is not a sin. Of course, it might not be entirely correct to be so, but it's not entirely avoidable either given the circumstances and different brought up patterns the partners hail from. Try to be calm from inside as much as possible when you talk to your partner about it. Be specific, avoid being generic as maximum as possible. Have an open and to-the-point talk. Don't take your partner for granted if he or she gois listening to you. Don't say anything and everything and then don't expect him or her to listen to you fully. It's not fair enough. You yourself might not be able to do it too, if you are in the receiving end. Try as maximum as possible to express your feelings than doubts. Tell him how do you feel along with why you do feel so. Make the discussion thoughtful rather than argumentative. Do not expect answers on the fly; allow your partner to have time that he or she might very well deserve. Believe that thinks will work out. Be patient and determined enough to get the solution for this problem.
Certain relationships in life are the beauty of human evolution. Be proud to be part of one such. Do your best to be true and dedicated to your partner. Set examples, positive ones. Live and let live!