Much as we wish we could, we can't dictate how our DC react when they are heartbroken. All we can do is be very loving to them and encourage them to be very loving towards themselves.
I taught DC the importance of always having several things in your life that are equally important so if one goes wring the others keep you going. Good friendships, a sport or hobby you really adore, a life ambition that is yours alone - not dependent on someone else doing something with you. It helped them, when they first had broken hearts, that life was going well with friends, hobbies, academic work etc.
Maybe you could help steer some of her focus towards other things that matter and that could raise her self esteem. Invite her best mates over for pizza and comedy film night (not rom com) or help her look for a part time job or work placement in some area that interests her, or get her new kit for a hobby or sport she loves.
Try and nudge her away from her phone, out onto a family walk and chat about the bigger picture - what sort of career does she want? What countries does she most want to visit? If she plays an instrument does she want to be in a band, etc. Does she think she'd like to go to uni or to work, once school is over.
Give her loads of treats if you can afford to, especially now she is revising for GCSEs. Maybe a new haircut or new outfit, remodel her room, take her out for dinner or breakfast or for mocktails, look out for tickets to a music or comedy gig she'd like and surprise her.
Even small things help. Run her a deep bubble bath and leave a mini tub of her favourite ice cream beside the bath. Make her favourite dinners. Sit and watch reruns of her favourite shows or films with her. If she is sporty, go for runs or cycle rides with her, take her swimming or climbing.
Just showing we care and subtly helping them realise there is a big world out there and they will soon be free to explore it is the best support we can give, in my experience. No point in trying to get them to forget him or say he isn't worth it - however much we want to. Just focus on other lovely things life has on offer.