Trust is an essential part of a relationship, and becomes an issue when one partner doesn't feel the other is being honest...
To avoid or cure this the solution is a "trust talk."
Other advices to develope trust in each other are:
Be consistent.
Be on time.
Don't lie.
Be fair.
Be sensitive.
Respect your partner's boundaries.
Be a good listener.
Try not to overreact when things go wrong.
Don't be jealous.
Don't dig up old wounds.