My fiancé wants to study abroad.

Hi everyone I need advice. My girl, wants to study abroad in Costa Rica for a month during the upcoming summer. This terrifies because I have studied abroad in the past, before I knew her, and I was even engaged to another woman at the time and the trip destroyed my previous relationship and ended it. Also, I have seen with my own two eyes first hand at least 4 other people that were in relationships also ranging from boyfriends and girlfriends to being engaged. And all of those relationships have crumbled also. The reason that I always have seen those relationships end is because while abroad one generally has little to no communication with your loved one because of lack of phones, the cost it is to make a Phone call when you finally are around a phone, and because your in a new country so usually you are out all day everyday exploring the new culture so you rarely get to a computer also. Further, during that time you are usually spending your time all day everyday with the same people of the opposite sex getting to know them quite well and building bonds with them and a lot of times catching feelings for them just because you share everything with them: all of your experiences, all of your feelings, all of your emotions. Things that you will generally share with your loved you share with people you are with because your loved one cant be reached because of the reasons that I mentioned previously.. And all of this terrifies me. My fiancé now we love each other sooo much and we both believe that we are sole mates for each other.. We have everything in common and share all of the same morals and values. She completes me and I complete her.. It is true love. But this trip I feel could end us because if she goes on it I told her I would have to end it because I am a very selfish person when it comes to my girls love and I don't want her sharing it with anyone but me and I don't want her building relationships and spending all day everyday with people of the opposite sex that are not me... Am I wrong to feel this way? She says that she wants to do this so she can put it on your grad school application to help her get accepted into occupation therapy graduate programs at a local university. I just don't feel that will be a deal breaker for her to get into the program because I know the exact emissions process at this local university because I have received my graduate degree from there . And because she is actively involved in 3 organiZations already, has volunteer work already, and her moms daycare that she helps out with full time. So that is plenty in my opinion to put on her application. So I gave her an ultimatum.. If she goes this summer when I can't go with her because I will be finishing up my classes to get into medical school I will break up with her . And I will do this for two reasons. One reason, is because we could easily do the study abroad together the following summer ( but she says she would love that but it would be to late to put it on her application for graduate school). And the 2nd reason is because I am sacrificing also for the relationship. For medical school I want to leave the country and go study at this medical school in Mexico because things that you do as a resident you do them as a med students in Mexico. It is a more hands on learning experience. But I am sacrificing not going and applying to medical schools here locally where we both live at so we want have a long distance relationship.. I am sacrificing what I want so why can't she do the same? I mean I am not even asking her not to go. I am just asking her to go the following year when we can both go together. Am I wrong to feel like this? Am I wrong to tell her that I will end our relationship if she goes abroad without me for a month because of what I have experienced with my own previous relationships and seen in other relationships how they all ended from study abroads????? Please help me