My ex boyfriend asked me for a break up which i gave to him because he was hurting me so much and i was not?

i was not happy.his best friend was there for me during that painful time of the break up and after 2 months he asked me out.i accepted because he is a nice and loving person than my ex. ithink i have falln for him. suprise! my ex is now after us and saying sorry to me and begging me to reconsider. what can i DO?