I get you, I would feel completely awkward too. I’m actually having a bit of sympathy anxiety here for you!
I do think the way you’re feeling is letting you know something isn’t quite right in this set up.
he may not have been unfaithful in the usual sense, but this secret friendship is definitely shady at best.
you said you split before as he was unhappy - do you think he’s feeling this way again and this is his escape or a little bit of excitement to fluff up his days? Even if it began in full innocence, it’s absolutely gone on far too long, and is definitely in dangerous territory now. They have had 18months of getting to know each arch other (even if it’s limited to just messaging) so there is obviously attachments formed.
in no way am I trying to make excuses for him here - he’s in the wrong and knows it too I’d say. 17 years is a long time and yes, we all know with that, kids, jobs, life admin and housework things can get a bit monotonous at times, and I’m fairly sure most of us have felt a teeny boost if someone has complimented us, but that’s usually where it does, and should, end. I can honestly say I day dreamed more than once in my relationship about running away!! 🤣 the difference is, we know what’s acceptable and what isn’t and make a conscious choice to respect our partners and the relationship.
again, I may be biased based on my own experience, but seems my ex thrived on a bit of female attention, sought it online, and then eventually took the plunge to cheating
I would be very wary of what both your partner and this woman expect out of this friendship they’ve created, and if it’s more important to him than your own feelings, and ultimately, your relationship together.
he cannot be that dense to not realise he is absolutely creating an issue here