Love it! So my dress wasnt a sweetheart but was strapless. It was a ballgown type on the bottom half. That being said how the top half fit determined how well the dress stayed up. I felt better on how the dress was secure with a strapless bra, yet didnt like the extra tightness going around my back because well these type of dresses are already fitted. I had it altered with the bra on but yet didnt really want to wear it. I found this bra on Amazon and it was perfect! I bought two sizes and then found out which fit best, then returned the incorrect size. Comes with bra and bonus items. The bra stayed put, provided what my strapless had, and I couldnt even tell I was wearing it. Just in case you are having the same dilemma in a strapless, hopefully it would save you the research! Also I have worn it since and its still staying well.
I really love this sticky bra. It's comfortable and stays in place.
I used one of these. I'm normally around a 42DD and ordered the E and F. E ended up working best for me. It didn't give me as much lift as I wanted, but it gathered my boobs together enough that the cups in my dress did the rest. I can't speak to how long it'll stick, though, as I eloped and only wore it for about 5 hours and didn't sweat much during. It's reusable, though, so you could do a test run if you'll need to wear it longer/might get sweatier.
I couldn't use tape because I had a plunging neckline and it would have been visible. These worked for me.
To be clear for OP, she doesn't have breast tissue. That's just fat. She uses a pair of silicone pads that clip together. They stick on your chest far apart from each other, then you pull them together and clip them.
https://www.amazon.com/dp/B075V53Q7S/
Then she sticks breast forms inside her bra.