Caught Myself Struggling

I just can't seem to shake off my ex-husband's shadow, but I caught myself the other day. Me and my new girlfriend, we were in her place for a weekend getaway. Her bed, our first night alone together after months of sneaking around at hers and mine. I was on top, her legs wrapped tight around me, eyes locked, breathing heavy. It was just about to get there when out of nowhere, the image flashed into my mind: him, that same position, but back when we were still in it together. The way he always had this grip, fingers digging into my hips like a desperate anchor. It felt so real for a split second, and I froze. My girlfriend was confused at first, then irritated, maybe even hurt. But after some explaining, she got it. She's seen me through a lot these last few months. We tried again that night but I had to pull out. Now it haunts me, when we're close and intimate, why do I still see him? It's not fair to her or me.