Shapewear

How backless are you talking? Do you have a picture? Depending on how low the back sits, a lot of shapewear won’t work out. Shapermint and Honeylove have shorts you can wear but anything covering the stomach area will be exposed with a backless dress. If you need breast lift/support, your best bet is Goodlines boob tape.

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It’s mesh. I think I need to suck in something for the tummy

Get something from the above listed stores and have your seamstress add a flesh colored panel to the back to hide the shaper.

My dress has a low back too. I ended up going with the “Skims Sculpt Low Back Short”. It was really the only thing I could find that was low enough. There is a lot of boning in the hips. A little coverage in the tummy. Better than nothing!

Check out Skims - they have ones specially made for low back and I've found their shape wear to be the most comfortable I've ever worn. I don't feel like I can't breathe or move in it, but still feel secure. They don't roll either. And they have a gusset crotch to make life even easier!

I think it may just be how you're standing with your arms in front. I see your shape! If you are interested, overhead shoulder presses, and upper back exercises will reframe your torso into a V, and straighten your posture elongating your frame.