Trust / Insecurity Issues

My girlfriend and I have been going out for 7 months (since summer of 2010) , and she lives in Brazil so we get to see each other twice a year if everything goes right, im 17 and she is 16, and i am her 1st boyfriend and note the fact she lost her mom back in 2008. In the summer we met and started going out and i stayed one month there with her, and we have kept the relationship going through video chat every day, but in the end of november, she said she started to get feelings for a guy she like back 2 year before, i was completely heartbroken and this discussion lasted several days and she cried and said sorry and everything, and then i went to Brazil a month later and stayed there 20 days, she swore to me she loved me, and that she was extremely sorry, she told me she figured that happened because she was feeling extremely lonely, i was able to forgive her but its been 2 months since this happened and it just haunts me, its horrible to admit but i feel my trust in her greatly diminished, we are church people so she always goes to meeting and rehearsals and stuff, she also lives within driving range of the guys she kissed once. From this distance i transmit to her a lot of insecurity, and i dont want to justify it. She loves me and i love her. I trust her but i dont trust the people that are sometimes around her, and im constantly afraid someone's going to take my place or she is going to end up falling for another guy since that guy might be closer, i dont know, im thinking of her all of the time, but she always reassures me that she only loves me and that she would never do anything to hurt me.

*I need some help, because i dont know what to do, i just want to be relaxed and not feel insecure, and i will not leave her because i love her