Well, first of all, don't listen to your boyfriend's sister. She is not in the relationship. She doesn't know what's really going on. Just tune her out. Easier said than done, I know. But really? Don't listen to her. Trust yourself.
My husband and I dated long distance for four years, like you, and we also survived him going abroad for a few months. Being long distance has made us a stronger couple. We know how to communicate really well. We also understand that we're better as a pair when we are strong individuals.
I can relate to the anxiety, the irrational voices in your head, the roller coaster. Focus on the good parts of your relationship. Keep pushing forward with your everyday life.
I hope you feel better soon.