I went through a very severe phase of this in my early 20's, ironically when I was at my fittest and healthiest. I was in a crazy place and my head was a wreck.I had a ridiculously intense relationship that I don't think I felt worthy of. Jealousy was a huge issue, and largely irrational too.

Fast forward to current times, after a 7 year break from relationships, doing things just for me I am now with a partner who thinks I am hottest thing to walk this earth, despite me being at my heaviest and 10 years his senior.
And frequently expresses as much.
So when he,sends,a picture of some sexy underwear on a pretty model, I don't even blink, because I know he's saying I want to see YOU in that.
Sure we both find the odd person attractive, but I don't ask if I don't want to know. It's human nature, hell it's in everything's nature to take note of something that attracts them. Quite often it's subconscious.
How many of us have watched a program and out of the blue accused our partner of being attracted to the leading lady, only to be met with utter bemusement, followed usually by futile denials or weary resignation, "oh no, here we go again."

It's a horrible place to be in, and for me it was directly linked to the subconscious belief that my partner was too good for me and would ultimately go off with someone else. This of course became a self fulfilling prophecy because my behaviour became too much to bear, for either of us.

Whilst I still wobble, my partners very expressive nature goes along way to stabilising those insecurities.
And it's the little things like never passing up a chance to compliment me, that help remind me he appreciates my worth.