First, make sure the other person is interested in me too and is available for a relationship. Talk it over with them and come to a consensus that I need to break up with my current partner.
Talk with the current partner and help her to understand what’s going on and why. Be kind, but firm.
If the other person is not available for a relationship or is not interested in me, do nothing because that’s not love. If infatuation with someone else were grounds for ending a relationship, then no one would ever keep a relationship for long.