I tried to manage it with diet too but it didn't matter what I ate my levels were always over. I was beating myself up about it in the biggest way as I really was doing everything I could but nothing was working. I felt the same way you did.
I had numerous meetings with the dietician, diabetes midwife and the obstetricians at the hospital before it was decided that diet simply wasn't working for me so I got placed on insulin. I was then able to eat (although still had to follow a strict diet), but even with the insulin my levels kept rising. This is not abnormal for GD - as you get further in the pregnancy so the placental hormones rise, and they adjust the insulin accordingly.
You just need to realise that some things are out of your control. You are doing the best you can, but sometimes medical intervention is required. And it is nothing to be ashamed of, or upset about.
If you ever want to talk about it or ask anything about my personal experience with GD and insulin/diet please feel free to PM me smile
Best of luck, hopefully the rest of the pregnancy goes smoothly for you smile