What is 'It's behind me'?
Rıza: You lost the race. How about winning in love? My marriage proposal is still valid.
Sultan: Had enough Russian salad, Rıza Kabakoz? Because when I left Mr. Pop Star wasn’t alone at the pool.
Rıza: Sultan, look
Sultan: I was drunk, I acted like an animal. Anything else you’d like to add?
Rıza: It’s behind me, I swear. Forgive me. Give me another chance.

1) Because when I left Mr. Pop Star wasn’t alone at the pool.
Is the 'Pop Star' here a man sings well and gets popular?
2) It’s behind me
Does it mean I don't mind it any more?

Question

What is 'It's behind me'?
Rıza: You lost the race. How about winning in love? My marriage proposal is still valid. 
Sultan: Had enough Russian salad, Rıza Kabakoz? Because when I left Mr. Pop Star wasn’t alone at the pool. 
Rıza: Sultan, look
Sultan: I was drunk, I acted like an animal. Anything else you’d like to add? 
Rıza: It’s behind me, I swear. Forgive me. Give me another chance.

1) Because when I left Mr. Pop Star wasn’t alone at the pool.
Is the 'Pop Star' here a man sings well and gets popular?
2) It’s behind me
Does it mean I don't mind it any more?

Answer

1.  Yes, a 'Pop Star' is someone who is famous for making music.  Usually a pop star is a singer, but it could also be someone who plays a musical instrument.  Some examples of pop stars are Madonna, Beyonce, Mick Jagger, Arianna Grande, Bono, Billy Joel, Shakira.

2.  Yes.  "It's behind me" means that I don't mind it anymore, or "I've gotten over it."   A similar expression some people will say is "it's in the past."

Answer

1)  It's difficult to say without context. A pop star is a famous person who sings pop music, however it could also be used sarcastically (for example Mr. Pop Star could be someone who thinks himself very talented).

2) Putting something behind you means you won't do it anymore, or you have moved on. If he says being drunk is behind him, he is promising to act differently in the future.