I see some of my life in your story, my ex husband and I (our anniversary also happened to be NYE too) were poly for about 9 months and then due to lying & cheating, we divorced. I was in my early 30's then and felt a lot of the things you feel. I am now married to the person I was dating at the time I divorced.
Really, being in your early 30's doesn't make you past your prime or unattractive. I am sure you know this but you probably could use a gentle reminder.
The one main difference for me is I stayed friends with my ex, so over the years we have been able to visit the things that went wrong, the lies, cheating, etc, and I think I have pretty much reached closure on them 7 years later, but it did take a lot of time, and negatively impact my relationship with my now husband, because I worried he would do the same things, hurt me the same way, and it was too easy to say "my ex did X, you did X, so I'm sure you will also do Y and Z and hurt me."
I wish I had some great advice, but I don't so much.
1. How do you work through issues that stem from old (but very deep) major hurts and insecurities from past relationships? My BF tells me over and over he loves me and isn't going anywhere, but I just can't seem to trust that now. How can you just "forget" about someone you supposedly love and care about? How do you move on from something like this and forgive? I feel like the act itself was so minor, why can't I just get over it?
I read a _lot_ of books. I go to the used bookstore and browse through the self help section, and grab anything that seems useful. Self esteem, self worth, communication, relationships, anything that strikes me as being useful or interesting.
2. How do you conjure back feelings attraction to someone, instead of being intimidated/threatened by them once a boundary has been crossed? I have no reason to be angry at K, but I am resentful of her now. Just being around her socially calls up huge feelings of inferiority, whereas I didn't feel that before. I hate feeling this way about myself and someone else. I want to feel good, but can't seem to pull out of this self loathing/depression.
I do not know. I imagine working on your self esteem would help though.
3. Is it sometimes a good idea to just push through something and do it, and face your fears and insecurities head on? Or am I just setting myself up for more pain and heartbreak? Sometimes I feel like I could handle a carefully negotiated scene with them if I forced myself to "get over it." But I also see potential for disaster there.
Sometimes, but you've already forced yourself to do two things you did not want to, and perhaps should not have (especially the first one).
1. not telling your boyfriend you did not want K to stay over.
2. going to the play party
Is there any chance you could play with K without your boyfriend? Do you think that would help at all? Maybe it would be good for you to remember that she is attracted to YOU too. If you don't think it'd be a positive experience to do so with your bf, I would really recommend that you don't risk it, and having to add another negative thing to what you are already dealing with emotionally.
I also want to point out you say contradicting things.
"I felt humiliated in front of everyone and disgusted that my BF would do something like that around my vanilla family. (I don't think they knew, but I'm too mortified to ask.) Either way, it was totally inappropriate."
"I felt like not only was I disregarded, but disrespected, and humiliated in front of our friends as well"
"I feel like the act itself was so minor, why can't I just get over it?"
DO you consider the act to be minor? I think because of the presence of your family I would find it hard to classify it as minor if I were in your shoes. I am wondering if you really feel it was minor or are trying to convince yourself to see it that way so you can get past the hurt you are feeling by telling yourself it's not a big deal . So the physical act is minor, but the emotional aspects of the act obviously carries a LOT of weight for you.
I also would have similar feelings and see it as indicative of a larger relationship problem, but I am sure my husband would say that it wasn't, if we were in this situation, and it would lead to lots and lots of long talks. I of course would overreact and go on and on about how our relationship was doomed and obviously we were totally incompatible, etc etc, so it is good you are keeping a level head.
"We've done poly stuff before, and I have always, always, always been super considerate of his feelings/potential feelings/boundaries."
Do you feel that he has also been super considerate of YOUR feelings and boundaries? Do you think that if you sat down and negotiated/re-negotiated boundaries that they would be respected and held to?
I know those of us who feel we are SO considerate of our partners feelings often expect others to show us the same behavior back and feel hurt when they fail to do so. It's better to remember that each person is different, and if you hold him to higher standards (of communication, considerateness, etc) than he can realistically give, it can cause resentment. As long as he is giving you as much consideration as he'd give to anybody else, then I would suggest to do your best to trust that he cares about you, no matter what your fear says.
Happiness will never come to those who fail to appreciate what they already have.