Oh OP, harsh truth time here, he doesn't want to be with you. It doesn't matter how much you love him, people need more than someone's love to be happy. Especially if he just does not feel that way about you. Your love alone is not enough to make a relationship last.
There's not always a reason why. Things change, people change, and even though 5 years is a long time, at some point he decided you were not the one for him. And it is better he ended it now rather than waste another 5 years of your life.
Of course, he won't block you. Just because he doesn't love you romantically, doesn't mean he has stopped caring for you! But this doesn't mean he wants to be in a relationship with you. Forget the other woman, she is irrelevant as even without her, he would still feel whatever gap there is, to make him break up.
Please stop messaging him, and accept that this is over. It will be painful, so allow yourself to grieve. And when you get angry or upset, call a friend or family to rant, rather than speak to him. As terrible as it feels now, you're only prolonging the pain the longer you stay in touch. So for your sanity, delete his number, block him (you can explain to him it's to help you move on, if you want), remove him from all social media (to stop you stalking and torturing yourself), and then throw yourself into whatever hobbies/friendships/distractions you can.
I'd also recommend reading the book, 'It's called a break up because it's broken' - it very funny and had great advice on how to get over a break up without losing your dignity.