i think these feelings are because you are on your own (i'm assuming) whilst your travelling and whilst you have a girlfriend you feel less lonely and isolated. when the girlfriend goes, its back to being on your own again.
could a bit of homesickness be causing this? or maybe you feel like you want to settle in one place now?
it can be exciting travelling to new places, meeting new people and having all these adventures (i went travelling last year), but in the end, i craved a more permanent setting (well at least for a little while, so i could catch my breath again)