Why did he leave?

I miss my ex so much, and I shouldnt because I discovered he had been seeing a lady at his job for 6 months What I miss is how special he made me feel when I was with him. At a point he lost his job and  was staying at his sister's for a while to get on his feet. During that time, I helped him get another job. From that job, he met another lady. His sister told him she was moving, around that time was when he took up w this lady, I didnt know about. He asked to move in w me. I said no, because he had some issues with substances that I felt he had to deal with. I had almost grown kids. Plus my landlord had a no smoking/ drugs policy, for which he smoked weed and drank a lot everyday.  There were times he came to my home staggering drunk in front of my kids, and that's a no no. 
The lady he fell for she is either a past/ present addict, because she had only half her teeth the rest are black, she is all tatted up, she has no hair, she has I'm crazy written across her neck. I dont get it. I am a professional who works very hard daily. Even with me being a professional, I never raised my voice or talked down to him. I never cheated. I tried to help him when I could.I'm not better than her, but different which is what he said he wanted. I was trying to give him space to get himself together and instead I get cheated on. He moved in w the other lady, no explanation. I just dont get it.