So you realize you are cheating on Tom, which I guess is some level of self awareness. Imo there's no separation of emotional vs physical and one is better or worse than the other. Unfaithful is unfaithful. You are deliberately engaging in romantic intimacy with someone other than your partner. Your own behavior has shown you are incapable of keeping things "truly platonic" with Kay. If you want to be in a romantic relationship with someone else imo you have to cut way way way down on time with Kay, perhaps even a period of no contact. And Kay has shown with her behavior she's probably not really ever going to take the plunge and fully and openly be with you. I even hestitate to call her your "ex" because yall really were never together by the info in this post. Yall had a messy situationship where Kay was cheating on her partner. Just bad all around.

Even if you ditch Tom and decide to keep the friendship with Kay what are you going to get out of that but more pain, longing, and frustration. Sometimes there's just people you can't be around because the timing is never right. It sucks but it's how we react to these times that shows our character. And right now both of yall seem to be so stuck in the weeds of your situation you can't zoom to see how it makes both of you behave in really bad ways.

Is Kay still with her partner?

it didn’t work out for them, she’s presently single

You might need to block her out. Otherwise, you might lose your boyfriend

You essentially were her side-hoe for all of this, do you understand that? For being so in-love, she chose him over you. How much more does that mean she cares for him? She got to have her cake and eat it too, as you were single in the beginning but she got to have you AND her boyfriend. Gross. And she knew what she was doing just as much as you do.

Kay was cheating on her partner to do romantic things with you, regardless of the lack of kissing/sex, and now you are emotionally cheating on your partner. This not a sustainable friendship with feeling like that present, but you seem to understand that.

A smart decision would be to be honest with Tom about your whole history with her. And I get the feeling you weren’t honest with him initially because you know the whole thing was wrong. Secondly, you should rethink your friendship for Kay.

If you married Tom, don’t you think it would topple his world to understand your feelings about him have no stability? Because you will be thinking of Kay in the back of your mind.

Exactly. Even if she dropped Tom… Kay already chose to not be with her. What is the point of living through the frustration? OP choose the person who will pick you first. Kay already chose who comes first in her life, and she didn’t pick you.

[removed]

That’s how I’m inclined to process this. It’s a hard truth to fully swallow.

You’re not wrong. As I read your comment, I think I’m making the realization that I’ve basically been in denial on multiple fronts. The truth, as you correctly point out, is that I’m not her priority, she did hurt me substantially already, and she will be the demise of my current relationship if things continue as-is. Thank you

You are welcome OP. I hope you figure it out and I’m sorry to lay it out like that, but it is what it is. Good luck