How can you tell if it isn’t meant to be?

In the beginning of my relationship with my boyfriend everything was perfect. It was like a movie. He was my Jim and I was Pam. Could you ever not want a relationship like Jim and Pam?! After I had our baby, it all went downhill. He was a crappy father and refused to help me at all. Of course I was super upset and having a really hard time. He refused to quit smoking weed, something he’d started towards the end of pregnancy. I caved and let him smoke ONLY before bed. I did not want him high around our baby. One day he came home, high as crap and I lost it. I told him to get out because I needed some time away from him. I had had enough of the disrespect and was at my breaking point. He was gone for three days before we sat down and had a talk and I let him back. He promised to change. He was helping a bit more and not smoking until bed. One night I went on his Snapchat and seen a girl I’ve never heard of before, so I read the convo. Long story short, they had swapped nudes and were flirting. I was completely shocked and devastated but decided to stay with him. About a month later, I was up late and he was asleep. His phone was beside me since both our phones were charging. He kept getting notifications so I checked to make sure it wasn’t his mom. I found he had an app he was using for months to talk to multiple different girls. Calling them babe and flirting. And I still stayed.. flash forward to now, he is a monster. He’s constantly mad, yelling, being a dick to everyone. He punched a hole in the wall because I tied the garbage bag before it was completely full. He broke our friends controlling because we went to get groceries without him while he was in bed sick. It was literally a ten minute walk there and back. He constantly says I don’t do anything and I’m lazy but I work, take care of our baby and clean the house ten times a day. I’m lazy because I ask him to do the dishes... He’s so greedy and freaks out if I don’t buy him something even tho I’m always broke. He has good qualities on top of the bad. Qualities I love. I don’t want to leave because I’m hoping he’ll change and he says he will but I’m tired of waiting. He smokes weed constantly now. His friend bought him a lot and when his friend ran out my boyfriend refused to give him any. I don’t know if we’re meant to be anymore. He’s not the person I love anymore. He needs help and I don’t know how I can help him change. What can I do?