You’re justified — but not realistic. 😉
Of course you’re upset, jealous, and may even feel betrayed. These are your feelings. You have them. They’re real.
However, the reality of dating is that it’s competitive, and it’s a process. You should use the first three months of dating someone simply to decide if you want to continue dating them, and during this time, play the field and assume that they are too. You wouldn’t want to declare yourself taken after two dates — because it’s too soon to know them well enough to make that type of commitment. And in fact, you’re learning about this other person who is still interested in playing the field and dating you at the same time — and it sounds like this is news to you, although not great news. You have competition whether you recognize it or not. And in this case, you do — whether it’s this one other person, or your date is interested in playing the field with lots of people.
I think you should consider continuing to date this person and be a little more careful with your heart while doing so. This was a good wake up call. But it does’t have to be the end.