I (23f) found out my boyfriend (24m) has been flirting with other girls through text

My boyfriend and I dated for almost 4 years. Today, I found out he has been flirtatiously texting other girls for months now. These texts include things like asking to meet up and sexual questions about them and he even told these girls that I broke up with him a year ago making them believe that he is single. I don't know how these girls are reaching out to them. I suspect it's through Tindr because last year, my friend found his Tindr profile, but he told me that the profile is fake. Someone has stolen his credit card around that time and he said they probably made a fake Tindr profile with his credit card information to buy all the premium stuff to chat with girls. I believed him of course because he had always been good to me and I trusted him 100%.

I found out about the texts because I was on his phone searching for something (It's a common thing for both of us to use each other's phones for years, though I only use his when mine is out of reach). I noticed a text popped up and I accidentally clicked on it which made me find out about the girl. Then I looked through his messages and found that he has silenced notifications for a number of girls on his phone which I thought was suspicious since he recently has also been defensive about me using his phone.

When I confronted him, he said that it was because he was feeling lonely, especially since I was working and going to grad school full time. Then, he admitted that he was on a lot of drugs and is having a drug problem which made him talk to all those girls and that it wasn't him speaking, but a result of the drug's influence. He's asking me to help him with his drug problem because I am the only one who knows and understands. He says I'm the only one he loves and that he even told his parents what he did (not the drug part though). I am having a lot of trouble trusting his words now and I don't know what to do. Please let me know what I should do.