How do i know if it is true?

i am in love with girl and she tells me she loves me i have only told one other girl i loved her and she cheeted on me and i am in the millitary and i want to know how to i let my self truley love her when i keep thinking that she is going to cheet on me but i know she would never do that kind of thing but i just dont know i still have that tought in my mind cause of the last girl that broke my heart i even asked her to marry me what do i do.