Cancer lied about having a gf !!!

I was dating a cancer for 6 months. At first everything was perfect. Saw each other every other day. Dates all the time. Texted all the time. Planned a family. He never used a condom and would not pull out. Lucky I did not get pregnant. He told me I was the only person he was dating but wanted to move slow into a relationship. I agreed because I've been hurt before. 4 months in I found out he was still on Tinder where I met him. He pulled away and we got into a major fight. But we tried to work it out. He still came over and we still went out. We still planned our lives. Right before New Years he got distant again. But he still kept saying it was work and he'd be back to his old self soon. I added him on FB and he kept flirting with this girl and that along with the distance I ended it again. He apologized but I was done. That was a week ago. So today I admit I missed him and went looking on his page. Found a girl and she called him "babe" went to her Twitter and boom. It's allllll about him. He was with her the whole time. Been with her for a year. And was distant around New Years cause that was her birthday. Now he's ignoring me. How could someone be so cruel? I've never known a cancer to have a gf and be this way. And why plan a family with me and try and get me pregnant if you have a gf? My mind is blown. He also convinced me to live near him so now I live down the street from him. Smh

Should I tell the gf? Not to get him back hell no. Just because it's so wrong what he's doing.