I need help on how to deal with the aftermath of a break up?

my cousin sister just got out from a 3 year relationship with her ex. she claimed he dumped her for another girl. an old flame to be exact. after breaking up with her, my cousin sister's ex got into a 1 month relationship with his old flame and got his sorry **** dumped after a month. immediately, he returned to my cousin sister and behaved as if he wanted to rekindle their lost relationship back. but he never asked her and this bugged my cousin sister quite a bit. so when she finally did pop the question as to why he behaved so lovingly and as if he wants to go back to things as they were before, he told her that he wasn't sure because he cannot help but feel less love for my cousin sister. shocked and annoyed by his response, my cousin sister left the city and returned to her hometown, as far away as possible from her ex. and now, she cannot help but feel depressed and constantly thinks about what could have been and what did she do wrong in the relationship. she's killing herself over it and i need some advice from you good folks out there on pointers to advice and help her through this. what do i say to her that would help her feel better?