Am I sinful for ending my relationship with a girl who persisted in forbidden phone relationships despite my repeated advice to her?

You have done well by cutting off your relationship with her due to her unresponsiveness to advice and her persistence in wrongdoing. There is no sin upon you in this, as long as the boycott is for the sake of Allah, because abandoning people of disobedience is permissible. The warning mentioned in the noble Hadith does not apply to you, because it is for those whose abandonment is for worldly matters.