Do i respond to this message from my ex boyfriend?

my ex boyfriend just texted me" i need to talk to you. i feel terrible" this is because last night he was a complete jerk. He had his ex sitting on top of his lap, flirting with her, and barely ignoliged me. I love him, but I have been getting hurt way to much now. We have been talking these past couple of days, on the phone at night for once I thought things were getting good again, then all the sudden the calls stopped coming in about 2 days ago. I need to know if i respond to this message. He promised me he had nothing for this girl ( this girl is his ex for a short amount of time)(she is older then him, 18). She is a tramp, and everyone knows it. I do not like her,and shes a b*tch to me. What he did last night hurt me, and I still love him and miss him but I want to know if i dont respond will he miss me? Will it make him want me more? Will he try again to contact me or do you think this might be it? btw- im 17.. me and my ex dated for 9 months, and we broke up about a month ago- we broke up due to trust over this girl. I do trust him- i heard rumors that were FALSE. All was great before that and were crazy about each other.