OK I’ll give my opinion on this. I’ve read all of this thread and remember your old ones slightly. I’m trying my best to put myself in your shoes.
Firstly- the phone thing. That’s not OK and you know it isn’t. The thing is, if there’s a niggle or a worry, you need to talk to your partner, be honest, be open and ask and listen to their version of events. Snooping is not only wrong but also in this case where there’s not actual evidence of cheating/ flirting, you start to paint your own picture. He’s sat in the dark, thinking things are OK whilst you’re snooping around, jumping to assumptions etc. I’ve had my jealous moments with my fiancé, I think we can all get worried or insecure at times- things have worried me but I’ll sit down and talk to him. I’ll ask him if I’ve got the wrong end of the stick etc. yes I’m not perfect, I’ve got upset etc but I would never snoop because frankly, that’s lying and I’d hate to invade on his privacy. You have to stop doing it and tell him you did it too. It’s important to me with my partner that if I’ve bothered by something, he gets to explain so we can work through the issue and carry on with our lives- in a transparent and honest way. I like to think If I’ve pissed him off and he feels we can chat about it, not start going through my phone.
The woman- I’m seeing this may be many don’t agree with but I actually do understand why a woman who is regularly messaging your partner who he, before you, had a sexual based, flirty friendship with, is making you feel rubbish. I’m trying to put myself in your position and I think yes, if it was a normal friend ( let’s be clear, I think we all know we don’t tell our normal friends that we wish we’d slept with them, want to tear their clothes off etc)- then no I wouldn’t care. One of my partners closest friends is a girl and she’s lovely- we recently went to visit her and her new baby. But that’s it, it’s simply friends. They were good friends throughout their uni life / 20’s and it’s just been based on normal friendship. I think if they’d had years of sexual tension and flirting then yes, I’d feel off about it. I think if I knew this woman messaging had only stopped the heavy sexual/ flirty messages because I was now in the picture, I’d worry of the intention- does she fancy him? Does she hope or think they have a deep connection and soon he will sway to her? But also, it could be fully innocent.
It also depends what he’s said. If he’s said to you, ‘ look, this friend I must admit, I’d flirt with a lot, we’ve nearly slept together, we had heavy texts but now that’s over, I don’t see her that way and we catch up’ then he’s been honest and it’s up to you whether you can be with someone who stays in touch with an old flame. You talk to him, not snoop.
If he’s told you that it’s just a standard friendship, no flirting, no sexual tension, no near miss sex yet you’ve now seen that’s a lie, then yes he’s lied and that’s bad on his part.
I do get your concerns but you need to talk to him, stop snooping and admit what you’ve done