I’ve been with my bf for nearly two years and we’ve always been good together. He’s kind and supportive and handles my issues pretty well. 
About a month ago I started talking to someone on Reddit and we still talk. I didn’t tell my bf until a few days ago. He didn’t like it but I assured him we were just friends and he’s okay with it. 
Now today I’ve woken up in the most horrible mood and when I was finally coming back from it again I noticed he was adding a coworkers number in his phone. He was struggling a bit with shortcuts so I helped him to open WhatsApp so he could send a message. As soon as I walked away though a very intense anger came over me and haven’t been able to let it go. My ex before him used to sext with random people and hide it and deny it and so this just triggered me massively. 
I feel like it’s completely unfair for me to be angry or hurt right now as he’s just talking to a coworker and he wasn’t even trying to hide it. Besides, I was talking to someone online and I didn’t tell him for a few weeks. I’m not really a jealous person usually (he sometimes talks about people at work flirting and that doesn’t do anything for me) but this just set me off. 
If I don’t try my hardest to shut up right now I’m just gonna blurt out something like ‘just go to her then and never come back’. 
I don’t know what to do and I can’t shed this anger. Help.