How should i apologize to my girl i hurt?

The reason why she didn't have interest in me no more was that I was acting like a kid she told me(i'm 18). I was just tryna express my feelings to her and she didn't understand what i said, i just wanted to know if she had feelings for me or this if this relationship is off. I guess it was my fault for acting "immature". I think i really f*cked up because i didn't trust her =(. I reallyl f*cking want her back, please help me what i should tell her. Don't flame please, remember i'm just an immature bas*ard.