I have struggled a lot with self-loathing.
I have not been able to internalise nice things people say about me. I think it takes a lot of practice. So I've got good at paying myself compliments 
...about the things that matter! I think I've tended to be praised for things that aren't relevant to my values system - things that don't really affect how I feel about myself.
Mona Pereth wrote:
What would you advise people to do instead of this sandwich technique? Would you prefer that the person just blurt out the criticism?
I prefer to be told things frankly

I can enjoy very critical feedback, unless it is said deliberately to wound, but if someone pads it for the sake of softening the blow it makes me cringe.
Personally I believe that the ability to simply and kindly state the unadorned truth conveys self-assurance.