sometimes you can't do much about it. but usually there are reasons for having trust issues...maybe you should dig a bit deeper in that topic, why she is having a hard time right now believing in you being faithful?
could be something she brought in from old relationships, where she made bad experiences or you act sometimes a bit "mysterious" in certain moments? maybe phone hiding or some stuff (not that you HAVE TO GIVE HER ACCESS TO YOUR PHONE, though!!) or you hang out with too many female friends who have a crush on you?
i don't know... just suggestions of possible reasons
could be something she brought in from old relationships, where she made bad experiences or you act sometimes a bit "mysterious" in certain moments? maybe phone hiding or some stuff (not that you HAVE TO GIVE HER ACCESS TO YOUR PHONE, though!!) or you hang out with too many female friends who have a crush on you?
i don't know... just suggestions of possible reasons