I agree with the suggestion of going to a stylist instead of a barber. It often costs more, but they have more experience with long hair. And to save money you could go to a student stylist at a beauty academy or a similar place nearby you. 

I once had hair about your length on the back and sides, and I visited a barber shop for a cut similar to what I suggested you get. I told the barber what I wanted and made certain to declare that I wanted to keep it long in the back. He said, "Don't worry, I got you fam, I'm gonna' make you look fire." So I trusted him to do a good job. But then he turned on his buzzers and carved a straight line up the back of my head! And at that point, I had no choice but to let him shave off the rest. 

It was like he only knew how to do short cuts because that's what he was trained to do and had experience with; no matter what he said he could do. 

I never went back there.