Dont try to improve your trust, can that even be done? improve your own self confidence.
doubting your partner is normally due to your own self confidence. my relationship started as a long distance one, we were young so i would work all saturday to afford to get the train down to see her (which cost as much as i earned!)
lasted for 7 years, ended today, i never once thought she was fooling around as our relationship was geniune and loving.