I was in a similar situation. The OW for 6 months then my partner and i started dating. I felt a lot of guilt and allowed the ex to contact me to vent and question...this went on for months but ive now changed my number so that she cant contact me as its had a huge impact on my relationship with him. She wasnt getting any answers from my partner so she resorted to getting them from me. Im an honest person but my partner isnt, he found it difficult to accept and hear the truth etc so i did that for her. Just tell her the honest truth, apologize and be sincere in doing so. It will help you move on too.