heartbreak

so it turns out my bf has been cheating on me. I saw him out in public with another woman hanging out with friends I didn't know, they told me she is his girlfriend. he didn't say anything and neither did i, I just left him there. its been a week haven't heard anything from him, he hasn't even tried to explain himself. this shit hurts because he was my first bf, first love, the only man I was ever intimate with. I am devastated right now. how can someone end something after 5years with no explanation, no remorse or feeling of guilt. I have known this man for over a decade and for things to end without a word as if I never existed is what's tearing me up inside the most.