My Boyfriend cheated on me for 2 years, with his ex.

My boyfriend and I have been together for about 3 years now. I love him dearly, I always have. He's an amazing guy and he's everything I've ever wanted. We are both in our early 20's now.
Recently I found out that for the first 2 years of our relationship, he was still seeing his ex. He had a serious relationship with her, on and off for about a year. But it ended really badly and he said he hated her. So I never, ever suspected anything. When I found out, I was broken. I couldn't believe he would do such a thing. He's brought up by his single mother and his gran - who are both amazing women. I know he knows how to respect women.
When I asked him how he could do it, he says he thought I would never find out.. So I asked but even if I didn't find out, how could he do something so wrong?? He says he wanted to hurt her in the same way his ex hurt him. He wanted to gain her trust and then break her..

I can't begin to fathom how evil that is.. And I just can't explain to you guys how much of a good person I thought he was before all of this.. It's absolutely crazy to me!!
So with deeper investigation I find out that he's been speaking to her for 2 years - telling her he loves her, he misses her, and even going to go see her. He promises that there was no sexual relationship, but I can't trust him like used to so I can't believe that.. I've asked him to explain why she was so important to him, and he says maybe because she was the first person he ever had sex with.. Which is understandable I guess.. but he dated her YEARS ago. And if he wasn't completely over her, why did he get into such a serious relationship with me?! We were just about to move in together. I was two days away from making the stupidest mistake of getting his initials tattooed on me..

I just need some advise on how to deal with the fact that I was giving him my everything while he was chasing after another girl.. I'm still with him, in hopes that we can make it through this..