desperation

Hi guys! I'm in such a terrible situation.
I and she were together about 2 months. She went out with some other guy, but said that she isn't interesed in him as a man. She liked his ideas and thoughts and she realised, that she want to learn some things from him. But I didn't trust her (asshole!), and after some time we quit.
Now it's 3 months gone. We didn't speak at all, but the last week we have met each other again. And now I have realised, that she is the only thing I need. But she's now dating with onother guy.
Have you ever been in situation like this? Can you give me some advice?
Thanks!

Did you think of her at all during the 3 months you 2 were apart?

Yeah, here's some advice.

Wait until she's single. Until she's out of that relationship, you'd be an asshole to try to get her out of it.