Shopping for bras online can definitely be tricky but hopefully I can help guide you to a new style that will offer excellent support and shape, it's amazing how much of a difference it can make! While we often recommend taking a trip to the store for a fitting, you can certainly try at home, just be prepared to try on more than one style or size that may not work.
I'm a little unclear if you're stating your under bust measurement puts you in a 32 band or you measure 32" around, so I'll try to answer for both scenarios.
For a 32 band I would start with either the Freya size 32FF or 32G. Keep in mind when you go down in band size, the cup size has to get bigger, so even an equivalent cup size to your old 36D would be a 32DDD, but that wasn't working for you so you'll need to go up a bit more.
For a 32" measurement, you should go for the 34 band size and I'd recommend Freya size 34F or 34FF.
The band on Freya styles may feel pretty snug to you the first time you wear them, especially if you're used to a 36 band. They should fit nice and snug on the loosest hook in the back. The band provides the most support on a bra so a snug fit ensures everything will stay in place and lifted.
Freya, Fantasie and Panache are all bras sold in UK cup sizing (hence the E,FF,G,GG) so if you fit well in a Freya size, I'd recommend sticking to some of the other styles sold in UK cup sizing. Additionally if a Freya F cup size fits you well, you may need to go up to a G cup size in brands like Chantelle or Wacoal.
While it may take a few tries, our free shipping and returns should make it easy for you to try on a several styles and ensure you get the right fit! If you find you have more questions, you can always contact our customer service team or one of our stores and speak to one of our certified fit specialists.