What would you do if your ex came back when you had a new boyfriend?

My ex broke up with me in January. I loved him more than anyone else I dated. I wanted a life with him. I was depressed for a while after the breakup but things got better. I have a new boyfriend and we’ve been dating for a couple of months.
Last week, my ex contacted me and told me he made a mistake in breaking up with me and wanted to be with me again. This aroused some of the old feelings again and I’m not sure how to respond to this. I still have feelings for him but I’m in a relationship and I always stick out a relationship to the end. I never cheat. I don’t know if I feel as strongly about my boyfriend as I once did for my ex.
I don’t know what to do. Should I stick with my boyfriend and see where things go? Or should I give my ex another chance?