Am I at Fault Here?

by DJR
(Boston, MA)

My ex dumped me earlier this past summer. During that time, I fooled around with this girl from work. We smoked twice w/ other people and we had sex once. I wasn't really feeling her like that so I cut it off. The same night I hit, i bumped into my ex at a bar and saw her with her girls/roommates, who all happen to me pretty promiscuous. I also walked in on her talking to some black dude at the bar (she usually dates black men and I'm black as well), who I thought she was trying to kick it with or vice versa. Then I left that bar with a few co-workers and saw her at some other bar with the same dude the same night. So I thought she was going to take him home or they were going to hook up. So this girl who I worked with over the summer was trying to get at me hard. She took me home and we did the "do". When me and my ex started kicking again, she asked if I had been with anyone else. I lied because I didn't want to hurt her and also didn't think we would be together again. We ended up going back out a little over a month later, but she found out like last week that I did fool around during our break-up period, so she called me a "slimeball", "piece of sh!t". I told her I was sorry and that I never cheated, but did lie. Now she says all she can see is me f'in that girl when she looks at me and now wants nothing to do with me. Am I really at fault here? I understand why she's mad but feel like she can't really hold that against me. Me being a guy and I thought she was taking that guy home. Help me please.