I kind of feel like this too. There's a room in his house I won't go in anymore because I found out it's where he lost his virginity to his ex. Luckily, it's not his bedroom, it was a room he was temporarily sleeping in because there was more space. The place I lost my virginity to my ex in is a hotel room so I never see that place again. I occasionally get horrible creeping feelings when at his place of "this bed I am sleeping in, she's slept here too..."
It's one thing I look forward to about him moving out this year the place will be completley 100% ex girlfriend free right down to new furniture! (this is his first apartment he's getting).
I'm not proud but I also did some online snooping, it was kind of self torture, I used to look at her pictures and imagine her with my boyfriend. I once (accidently this time though) found an abandoned blog which was in her language so I couldn't understand it, except for my boyfriend's name with hearts next to it and romantic icons. That made me cry. When he found out about it he actually messaged his ex and asked her to take it down, which surprised me, but it's sweet.
It's horrible, I even get the creeping feeling when he talks about things unrelated to her that happened during the time he was with her. Like his holiday to Thailand, he didn't even speak to her while out there as he couldn't afford the bill, but I look at his holiday pictures and think "was he thinking of her when this was taken? Was he missing her?". It's hard when his parents tell me stories from that time, I mean there's one that's a funny story but it still stings because it took place when they picked him up from the airport after he was visiting her. They don't really know how I feel about his ex and I don't plan on ever telling them. I am just glad they never really knew her well, and know me a lot better.
But the creeping feelings have gotten a lot more controled, and my boyfriend knows of my problems. Basically whenever my mind starts creating crap about his ex I'm supposed to talk to him so he can set it straight...as he says, the images in my head are most likely a lot more romantic and sexy than the truth. In my head they shared a love that could inspire poetry and novels, wheras he says their relationship was a simple teenage thing that he thought was love but only because he hadn't encountered the real thing before. By him killing the images and reminding me that I am important to him it helps me control it. The feelings have not gone away, but they have grown more manageable and less frequent.