How does one learn to trust (him) again?

A couple of months ago I found out that my boyfriend of one year had been cheating on me throughout our entire relationship with numerous random women he met on the internet. We were one week away from moving in together at the time and he was planning on proposing.

I found out from one of the girls he'd be sleeping with, who discovered, through personal research on facebook, that he'd been lying about being single. I then looked through his phone and found texts he'd been sending to other women arranging hookups and stuff. There were 4 women in total that I know of, but I'm sure there were more.

I dumped him right away but after talking everything through, he let me in on things about his past that I didn't know and he told me that the reason he did all that he did to me was because he loved me so much it scared him and since I'm 10 years younger than he is, he was afraid I'd grow bored of him and leave him so he kept those women on the side as sort of a 'security blanket'.

Now, I'm not stupid. I'm not naive. I know how lame his excuses sound but I thought everything through and based on all that he told me I went against what I thought was right and what everyone else had been telling me I should do, and decided I'd forgive him and try to rebuild a relationship since I love him.

....But the trust is gone. No matter how many times I tell myself that he's being genuine and I do believe that he is...I just don't trust him anymore.

My question to you, Reddit, is...based on your personal experience, can that trust be re-built in a relationship? I can't get all that stuff out of my mind. Those texts. The emails that him and that girl exchanged. The mental images of him screwing those other girls. All the lies he told me. The deceit. Can I get over that? /Should/ I get over that?