What would you do if you were in this position?

Take your time before you decide what to do. This is a very complicated situation that you are in. Will your ex leave again? If I were you I would follow my heart and make it work

you should end a relationship because you feel there is something wrong with it, not because something better has come alone. also when your dumbed you should wait long enough to get over the old girlfriend before you start a new relationship. (which clearly you didnt)

A relationship is not like a car where you trade one in, when a newer better model becomes available.

Normally Id say don't do it. She dumped you, why do you think it would be any better this time?
but
she left you to start a new relationship, you want to leave your girlfriend to be with her. go for it, you two are made for each other

Yes I will surely but not before speaking out not to do the same mistakes done earlier . and find and tell the expectations between both of you . also discussing the future plans of each will help to see if you both can be together for long