it will be difficult. but to get over you have to concentrate more on yourself, build hobbies, go out to trips, talk to new people but the most difficult part would be to stop thinking about him/her. you have to stop looking at old photos, memories and all . make yourself busy but eventually you'll get over
I am not sure..but feel like I became 3rd party in my relationship. i know relationship not possible forcefully.
Thinking about let them happy.. Silently vanish from them ..this 2nd time happening the same.
clearly it's their loss. you feel so much for him/her. Try talking to him once. If he wants to be with this new girl let him be. you'll definitely find the right person.