Hi guys, sorry for any formatting issues, I'm on mobile and not sure how to add flair to this (sorry mods!) So as the title states I'm in really desperate need of some advice. So my (on and off again) s.o of about 4 years and I were in a relationship for a about a year and officially broke up about 2 years ago but continued seeing each other and pretty much acting like a couple except there was no label and we technically weren't exclusive. 
This was all his decision as he did not want a relationship. I have tried multiple times to get over him as I find him a huge trigger to my BPD. But because of my intense feelings for him have been unable to do so. 
He is leaving the country on Friday to go to America and then the UK for a few years so obviously we are finally calling it quits. I say goodbye to him tomorrow and I can't think about it without breaking down crying, I can hardly hold myself together and am feeling trapped and out of control and to be honest I'm really afraid I'm going to do something dumb when he leaves.
Have any of you guys had to deal with someone important to you all of a sudden not being a part of your life anymore? What are some ways that could help me deal with this? Thanks guys.