Seeing someone new

Cassie • 20 years young • Australia • Taking it as it comes
The guy I was with for 8 months as seen in the posts who I stopped seeing officially just a month ago is seeing someone else now. He was without his phone for 2 of those weeks and wouldn't have been able to communicate with her anyway so it's either been going on for a while or it's still relatively new since he only came back Friday. 
I feel a little sick to my stomach because it's like, were you just using me for sex and someone to talk to cause you were bored? How does someone get over someone they supposedly cared about so much, so easily. 
Does anyone have any advice on how to get over this?