I'm no psychologist, but I would break trust issues into 2 categories: rational or insecure. Rationally, a girl might have trust issues with her boyfriend if he goes out alone with another girl regularly. An insecure girl might not trust her boyfriend when she sees him talking to another girl. There is a lot of gray area in between so don't push your luck, but Rheada's 2 rules really address both issues. Rule #1 for avoiding rational trust issues, and Rule #2 to lessen insecurities.
Speaking more toward insecurities: many fail by trying to "fix" the other person's insecurities. "Baby, I love you, I would never cheat on you. You gotta trust me." may appear to improve the situation while there is a sense of connection and intimacy, but the issue remains (or worsens).
Imagine if that person had said, "Baby, I'm sorry that I'm not making it easier for you to trust me. What can I do to help? Can I call you twice as often or answer your calls more often, can I share more details about what I'm doing all the time? Do I need to stop hangin out with certain people or be more transparent about what I'm doing?". This is about accepting (not changing) that she may have trust issues, loving her still, and making yourself more trust-worthy. Then, trust can grow over time(months and years). This isn't a "10 easy steps to trust", or "gain trust in 30 days" - this is a lifelong battle - you can fight through it together, or you can part ways like most relationships.
And if the reverse is a problem (i.e. you don't trust her) then she needs to know why, and what she can do to help reassure you. Communication (with action) is gonna be instrumental. Just know that distance doesn't cause mistrust, it fuels it. Be glad that the problem is obvious now, instead of a few years down the road when the stakes are higher.
-Anthony (3 months before my wedding: a 3-year, 2000 mile relationship)
A long distant relationship is very hard but it is possible. To stay committed and trust each other you will need to follow a few rules. One, stay away from any temptations. Don't hang out with people that are living the single life. And secondly, even though you are not together you can talk daily, email each other, leave messages for each other and even have surprises delivered to each other. This will keep the relationship alive and interesting.
When both of you do get together make the most out of the time you have together to make great memories.
It will take work but a long distant relationship is very possible.
I agree with Rheada. Simply ask yourself, would I want her/him to do this, write this, go there, etc... Although emailing, and phone calls are wonderful, it doesn't take the place of the human touch. With both of you understanding that, you simply make rules, and follow them to a "T" as if your life depended on it.
When you are able to be together, just think of the trust you will have created, and how your relationship will have benefited; From both of your committment to each other, and the strong foundation you have created for a life time of love.
Bottom line? You both have to be 100% committed. Rheada is correct that it will take a lot of work; But isn't she worth it?
Try working out solution or if you havent already found them to try to spend more time with each and depending how long you have been dating and how far you are I would also suggest moving in with each even if it a scrafice and you really love each other . One of you will be willing to make that happen.