Identify the problem
If you are not aware of the problem then you must work hard to identify the root cause. There can be multiple reasons for the trust issues, so you should cater all the aspects. You should take a paper and pen and start jotting down all the points you deem necessary to be addressed. If you have done anything wrong to upset your partner, you must write it down and if your spouse has done anything like that, then you should also list own them.
Tell your partner about removing differences
In order to fix the relationship, you must tell your partner about your consent and the efforts you are trying to make to fix the relationship. You must enlighten your partner about this in great detail, as this will also inclined him/her to make efforts and he/she will be motivated to fix the relationship.
Apologise to your partner
You must apologise to your partner, telling her/him about the mistakes you made. Note that saying sorry does not make you small but this small act would play a considerable role in fixing the relationship. This will lighten the heart of your partner, as she/he will believe that you are asking for her/his forgiveness and determined to keep things in perspective.
Explain issues to your partner
If you have any issues from your partner or you seek any type of help, you must tell her straight away. Do not feel shy and let her know if she/he has done anything wrong or if with a little extra effort, she/he can help sustaining the relationship.
Of all the points, the most important one is to change yourself. If you apologise, but do not change, then the things will again go down that track. Therefore, it is of utmost importance that you prove yourself to your partner.