Should I come back to my girlfriend?

So I have been in a 3 year relationship with my gf, and last month we broke up due to internal problems and fusions in some arguments. We still texted each other and hanged out when there were more friends with us and even nobody knew we broke up. Last weekend, she got out with a guy almost 20 years older and they got together as she tried to “forget” me by going out with him. Instead, she just understood that I am the one for her and rejected that dude and now wants to come back to me. I kinda feel betrayed by her, because I didnt wanted anyone else than her and was pretty sure we will get back again. I dont know what to do, any opinions?
Thanks