I’m going to stick my head on the block here OP and say that this used to be me.
I was incredibly insecure, still am a bit truth be told, and I just couldn’t shake it in spite of being with my DH for nearly two decades and having two DC!
My turning point came last Christmas, when I checked DH phone after his Christmas night out and found a confirmation for an order for my favourite perfume which I knew must be my Christmas present, and it was - I suddenly thought “what the bloody hell are you doing sugarcoatit?!”
I never once found anything remotely interesting or red flag in all my time of checking and I’d feel awful afterwards.
I took a long hard look at myself.
I realised that my first boyfriend, in every sense of the word, had left a huge hangover from when he decided to go behind my back with one of my best friends - a double betrayal.
He wasn’t a patch on DH, and I’ve no feelings for him, but it obviously knocked me more than I realised and left a big scar.
DH is also very attractive, I’m pretty in my own non conventional way, but let’s just say I did very well for myself. (Although he’s always telling me how beautiful I am, and I like to think that I nail things personality wise!)
I always used to think he could be with someone way prettier than me, but what I came to realise and finally accept was - you know what? He chose ME.
I used to be a very confident person, life’s dealt me some knocks, and it’s easy to lose yourself along the way.
I came to realise my behaviour was self destructive and potentially destructive to our relationship and so I sucked it up and started to work on myself.
I’m still insecure, but I no longer try to self soothe by checking up on my DH.
I strongly urge you to stop this destructive behaviour and to sit down and take a good hard look at yourself, try and unpick your behaviour, see what’s driving it, and work on yourself and your self esteem.
You say you wouldn’t mind your partner checking on you, but you are just trying to justify your actions to yourself here - at the end of the day, it is a gross invasion of their privacy and a betrayal of the trust they’ve placed in you and the faith they have in your relationship. It is undermining your relationship and eroding your connection.
There are loads of great books out there, and CBT is great too.
I hope you can take this advice and empathy on board and move forward into more positive behaviours and better coping strategies.
Wishing you all the best.