I was a bit of a sugar monster too (gluten wasn't that much of a big deal for me as I gave it up years ago anyway - still have white rice every now and then though). I could happily eat all the chocolate in the world!
The best thing that helped me give up sugar was fat and going cold turkey. I completely cut out sugar for as long as possible (well a week) including sweeteners, fruit etc. and any time I felt that I "had" to have something sweet I had something fatty instead (usually double cream / heavy cream). I now only have sugar once a week when a friend brings a gluten free cake for us to share after dinner and I have really noticed the difference (belly is shrinking much quicker than it was when I was primal but still a sugar burner). No longer crave sweet things which may be down to much more stable blood sugars.
However, I think the best way is to tackle one thing at a time but thats just my opinion. As Smartuko says, cutting it out completely works too - just depends on the person you are. I'm one of those people that, if I think I am depriving myself of something will go and eat it just to prove I can (I'm weird like that!) so the stealth method worked for me. Maybe start with grains and get that dialled in then tackle the sugar (or visa versa).
"Every day do something that will inch you closer to a better tomorrow."
"What progress, you ask, have I made? I have begun to be a friend to myself."
Hecato, Greek philosopher
Take the first step.
Even if you cannot see the whole staircase,
Just take the first step.