My wife had trust issues when I met her and they continued on into the first year or two of our marriage. I knew they weren't an aspect of her character, that she was inherently mistrusting...because she had and did trust others quite easily. It was part of the damage done to her in her previous relationship...her ex had repeatedly broken her trust over and over again. It had become more and more ingrained in her mind over those three years that she couldn't trust the guy she was with. To say that this distrust bled into our relationship would be putting it mildly...I could tell some stories that seemed insane at the time and now are just funny.
So obviously, since I love her and want our marriage to work, I wanted to resolve these trust issues. Her behavior was this: become angry and hold it inside, explode at me with some baseless accusations when I tried to find out why she was angry, then stew in her anger when I refused to admit to whatever wrongdoing she mistrusted me for. I wouldn't apologize for anything I haven't done...though I will apologize for mistakes I make. This would aggravate her. If I became angry at her accusations, it was if I were adding fuel to the fire. So here's what I did...
First, I had to make sure she had no reason to distrust me. No secret email accounts, no locked boxes, no hidden anything. I wasn't doing anything I shouldn't have so this wasn't really hard...but it was important for her to know I had no secrets to hide. Second, when I sensed she was angry at something...I made her explain it. Instead of getting angry at her, or telling her how crazy the accusations sounded, I simply told her how much it hurt that she didn't trust me. I let her know that I knew it was because of what happened to her in her past...and that I'd help her get through it. I told her I'd earn her trust if it took the rest of my life. Then I'd usually leave the argument there...I'd ignore her if she still tried to fight about it. If she calmed down, I'd go back to spending time with her...giving her my love and affection.
It only took a little while before she really turned the corner. She realized that all the time she spent angry at me was wasted and only hurting me and herself. She realized it was all about her insecurities and nothing to do with anything I did. She ended up letting her guard down and opening herself up to being hurt by trusting me...instead of just not trusting me and expecting the worst every time.
She trusts me completely, as I do her. We hide nothing from each other, yet neither of us feel the need to snoop into phones or emails or anything really. When she realized how much she'd changed, how I'd helped her, she almost cried because of how much better it was to trust each other instead of the fear, anger, insecurity, guilt, and all the other awful things that came along with the mistrust. We even joke now about some of the things she used to accuse me of and get mad over...though I think she's a little embarrassed about some of them.
So the point here is that it's not necessarily bad to try and change something about someone, but be sure of the reasons you do it. I didn't want my wife to trust me so I could get away with things I shouldn't be doing...I wanted it to stop hurting her, and me, and our marriage. I wanted her to be happy, and I knew that couldn't happen without trust.