It hurts to feel you can be replaced so easily. Your reaction is 100% understandable.
What to do?
Seek solace in work or school, your hobbies, your friends. Make yourself busy, whether it's to hook a rug or complete the big work presentation early. Be so busy that you get into the zone and don't think about him or this. I can't promise you'll never feel bad about this again, but you'll at least minimize that time and won't be wallowing in it.
And, next time, don't allow yourself to fall into the FWB trap. I keep watching people coming and going in here who are in FWB situations and, inevitably, somebody stops feeling it's a throwaway, and starts to believe it's something more. This uneven disconnect makes people miserable, and it screw with their heads.
I am not saying you need to go back to 1950s style dating but seriously consider having a relationship where you are courted or you do the courting, where there are dinners and movies out, and you have activities where your clothes are on and you actually meet each other's families and friends. And while that may or may not be love, it will be a far better way for you to treat others, and for you to be treated.