If one person in a relationship has self-esteem issues, they may be unable to trust their partner. Accusations of cheating are often one of the tools they use to drive the other person away, and it does not matter if their partner is faithful or not. The person with issues convinced themselves that their partner cannot possibly want to be with them, and they then work to end the relationship by blaming their partner.
An understanding person may be able to help their partner overcome this type of issue, but both will have to put in hard work to solve the issue. The person who has the problem must learn how to trust, and their partner will need to help them learn to be confident in who they are and why they are worthy of love. Few people in today's world are willing to invest this much work in another person, but those that do will have a strong and healthy relationship.