Ladies, How to politely ask and convince my wife to wear thong bikini on beach (please read details)?

Ladies,

My wife and I have both 26 years and we are together for 3 years.
She is very sexy to me and when I met her was really sexy and attractive girl.
Now she gained few pounds. I really don't care about that and i tell her everyday how sexy she is now and how much I like her belly.

She was very free, courage, sexy and confident lady before. She always dressed decent but sexy and guy was turning their heads after her. Now she thinks she is fat, even if she really isn't fat at all, she just has a small belly. Now she dresses only baggy and big clothes.
I would like to make her dress sexy again and be more confident like before.

When we talked about vacation last year she told me she would like to wear thong bikini and was shocked at first (also a bit jealous), especially when she told me she already did that before when she was younger. As I started to think about that I became more and more interested, even obsessed with that idea. I really liked all that and imagined her many times. She put that seed in my mind and that idea was growing very nice and fast :)

When I told her now how excited I am that we finally can go to that vacation and how it turns me on to watch her in that thong bikini and how much I will enjoy her body, she turned me down telling me she doesn't want to wear that anymore, she doesn't feel sexy as before and she is not pretty enough for something like that.

How can I politely ask her to wear that bikini? How to make her be feel sexy, desired, pretty and confident again?
How not to be rude but discuss with her about her insecurity and my desires that she dress sexy again?

Any help would be appreciated and thank you in advance ladies.
Ladies, How to politely ask and convince my wife to wear thong bikini on beach (please read details)?
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