bad boyfriend

Shae
About 2-3 months ago I found out that my boyfriend had been cheating on me for about a month with another girl, I was so heartbroken to know that he'd been ignoring me for her. 
When I questioned him he was honest and told me everything but he always blamed it on drugs. 
I don't know if staying with him was the right choice as I'm still finding it so hard to see past the lies and the heart break. 
I do truly love him and we wanted a future together but I'm worried that me forgiving him now means he'll just do it again... was I right in staying with him or should I have left the day I found out?