Advice on coping with infidelity. Help!

Last night I found out my boyfriend had sex with his ex the night before and last Sunday. I found this because I saw his ex's email in his inbox and emailed her back saying, "Stop emailing my boyfriend." She wrote back saying, "if he is your boyfriend, why did he come to me to have sex?" and so on. I confronted my boyfirend and he admitted. Now he says very sorry for what he has done, but he still loves me, etc. He says he did it because he was weak. (I don't buy it at all.) I loved him tremendously and I really wanted to marry him. So you can imagine how devastated I am. We've been together for one and half years. I never suspected any cheating until yesterday. It just so happened last night, so I cannot say I no longer love him. But I'm not sure even if I go back to him, I will never be able to forgive him. Then we will never be able to be happy again. I think I will keep bringing this up again and again whenever we fight. I've been in bed this Sunday afternoon, not knoing what to do. Tears keep coming out. Any advice for me please?

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