Extreme Guilt After Eating High Calorie Foods

Hi Reddit. I just want to start this out by saying I have never been diagnosed with an eating disorder, and I may not have one, but I felt like I needed to talk about this, and my coworkers didn't get it so here I am.

About four years ago I met my (now) ex husband and through our relationship I gained a lot of weight. I'm 5'6"ish and weigh 220, I used to be 150 and pretty fit. I'm single now and trying to get healthy and lose the weight. I have a gym membership and I go 5ish times a week. I have a fitbit, I track what I eat. And today at my office, for administrative appreciation day, my boss ordered pizza. I had four slices of medium veggie pizza and the guilt/anger/flood of emotions that followed was earth shattering. I got so mad at myself and started telling myself "this is why you gained weight, why can't you show some self control, this is why you are fat" and started thinking about how tonight at the gym I have to make sure I burn off the pizza calories (according to myfitness pal I'm still within my goal for the day). And this isn't an isolated event either. I try really hard to eat right and I did lose about 8 pounds in the last month or so, but I was literally so angry at myself today for eating pizza with my coworkers that I was in tears and yelling at myself. I tried explaining to one work friend that I was upset and they didn't get it; she just told me I look fine and to stop being silly. I don't know. I guess I just felt like I needed to vent. Thanks Reddit.