Help with soon to be long term relationship?

My girlfriend and I have been together for 11 months and I love her tremendously, but next year she leaves for college and it'll be 2 years before we can actively visit again. She is faithful and loves me equally but sometimes drops into depressions moods that last no more than a day. We've been through so many good times together but I dont know if she can make it. She lives in Highland Park (rich Dallas community) and I live about 40 minutes from her. Her adoptive parents were and are abusive (not physically) and she was emotionally isolated a lot in high school and has some self esteem issues to, but whenever we're together she's as happy as can be...but now she's almost crying at the moment and I dont know how to continue dealing with her moods. Its like whenever we're apart her mind feeds her "whatifs" or something bad happens. Any help is appreciated, sorry about the long-*** backstory.