Lost and confused....

So I'll trying and make this as short as possible as it's really long. So me and my fiance have been on and off for 10 years. We finally just had our first baby together last year and it was seeming to be going well. But the last few weeks I feel like hes pulling away. Not talking to me as much I barely get a hi and he's always on his phone. Hes being saying hes working late and more but it doesn't show on his check and I found out that he got drugs off someone but I don't know if they are for him or someone else. So with this all going on someone I use to mess around with when we were off came back into my life this week and I'm feeling a how bunch of new feelings. Hes been telling me he wants me and that he loves me and wants me to leave my fiance but I'm just too confused to decide that right now. Yes I do love my fiance I just feel like maybe he isn't the happiest anymore. I just could use some advice please. This stress has me not sleeping.