Vacation blues (PLEASE HELP AND READ)

I have a boyfriend of 14 months, and we are very, very serious and committed. However, his last vacation he went on with his family, he found another girl, and talked to her and "enjoyed" it while we we're dating. He did this behind my back, and he met here there. That was in November, however hes going again on the 24th, of this month. (March) and I'm terrified. Whenever I talk about it he tells me I need to trust him and that I should not worry.. But I am terrified, I have anxiety and I need help on what to do to handle this better. Please, please, please, please, help me and comment some other things to