Bra help needed please

[thesurreyyouth]

Following the bra intervention earlier this year I measured myself at 29.5" and 35.5" so changed from a 34C to a 30F (debenhams) 30E (M&S).
The band feels great but I'm unsure about the cup size. I've recently bought & returned 30DD which gave me 2 extra top boobs & made me feel very fat.
The 30E doesn't fit well at the top near the arm pit where it joins the strap, lots of gaping space. I also feel as though my boobs are too close to my armpit making me feel very wide and matronly.
Any ideas where I'm going wrong? I'm shy about trying on bras in shops & get flustered by offers of assistance as I'm not very body confident so prefer to buy & try at home later.
Thanks

Have you tried a 28 band? I have the same underbust measurement and first bought 30 but after a while it loosened. Going down a band size to 28 and up a cup size might help.

I'm not sure of trying a smaller band. I don't understand how a DD leaves me spilling over the top but an E has a lot of empty space. I struggled to find a choice of bras in 30 & the sales assistant made me doubt I had the right size by querying the band size.

I don't understand how a DD leaves me spilling over the top but an E has a lot of empty space

Sometimes it's to do with proportions and the way the band sits. However, I suspect this is more of a style issue. I measure 29"/29.5" under bust and usually wear a 30. Ignore the sales assistant

Which bra is it? Is it a moulded t-shirt style? They are very tricky to fit and many people find that they gape.

Thank you. Its a plunge moulded padded underwired bra.
Similar to this m.marksandspencer.com/mt/www.marksandspencer.com/flirt-lace-padded-plunge-dd-gg-bra/p/p22189479?jsrl=stop&un_jtt_redirect
The cup feels empty under the arm & gapes at the leading edge at the top.
Any ideas on a better style please?

Try a non moulded balconette from somewhere that isn't marks!

Moulded bras are very difficult to get a good fit with as the shape is so inflexible. Going from overspill to gaping is really common.

Something along the lines of the Cleo juna, Cleo marcie, Freya patsy, bravissimo half cup styles etc might be better -ie a wider, squarer neckline with less fabric at the top outer edge of the bra.