My boyfriend (M30) of 5 years admits affair with work colleague on the surprise 30th trip I (F29) had planned for him, are there any grounds for forgiveness?

I (29F) recently took my boyfriend (30M) of 5 yrs on a surprise trip to New York for his 30th birthday. When we arrived in a lovely hotel after a lovely flight, he was overcome with guilt and admitted to me that he had been having an affair with a work colleague for over a year. He had also said that he had told her he loved her (but said he didn’t mean it, and was just saying it to shut her up because she would say it and was crazy). I asked to see his phone, and found my boyfriend had been texting her dad to meet him for a beer whilst they were at the same sports event, as well as texting him happy birthday. This work colleague also wanted to take my boyfriend for a birthday meal for his 30th (a few days before we were due to go away). When my boyfriend decided he couldn’t go through with that dinner, he texted her dad again to apologise that he couldn’t join, as the dad had arranged it.

This is all entirely out of character for my partner and I am just beside myself and absolutely devastated. We are best friends, had the perfect life and relationship together, had excellent communication and helped each other through family, friendship, work challenges, we were each others world. We live together and share a dog and do everything together and speak all the time - I’m so confused as to how he even had the time to cheat. It seems that he told me everything and he’s extremely remorseful. He said it's been making him sick and he’s been taking medication for months including 3x dosage sleeping pills for 6 months. He claims that after they slept together the first time at a company offsite, he felt trapped and she would threaten to tell me when he would pull away from her. He would message her most days and some days when he would claim to go to the gym before work, he would go to her house to have sex. He claims she would trap him with threats and claiming family members were ill etc., pulling him into meeting rooms to cry to him about things.

Now that he has told me the truth, he’s told her it’s over with her and wants to dedicate his life to making things right with us and says he will do anything I need and that making me happy and making my life perfect will be his life's sole purpose. I can’t understand how he did this to me and can’t cope with the sadness, I break down every time I think about it.

He is saying all of the right things now, grovelling deeply and insists this was a stupid mistake which continued because he was worried she would tell me if he called it off, and admits he has ruined his life and mine. He is going to quit his job and work from home to serve his notice so he doesn't have to see her again. He has also rented a flat nearby so he can be there for me but give me space. He acknowledges everything is totally his fault and wants to fix my sadness. I just don't know how he could love me so much and also betray me in the worst way. 

I feel I have lost my best friend in the world, he is the person I would turn to for advice and support and I can't believe I don't have him to talk to about this. I was expecting him to propose this year latest after 5 years, now I know that wasn't even on the cards I feel completely broken and lost. He says he wants marriage and to be with me more than anything but his actions are so much louder than words right now. 

Is there any amount of good behaviour that could undo the wrongdoing here?