nycindie - totally agree. I'm getting better at this overthinking thing, though it's slow and probably not obvious from my posts. You're right though - its not really the polything that's the root of the problem, it's me. I've been making passive/aggressive attempts to make this relationship my ideal one through controlling her, and its not healthy. Granted it's not THAT much before this poly thing, but that's what it brought it to my awareness.
Right now, I'd say the REAL issue comes down to now that my girlfriend has something going on her life she is not 100% comfortable sharing with me. That's something new to our relationship. I would like to think that between my talks with her, and the responses I've gotten to my thread here, have helped me realize THAT IS OKAY.
Small update - S and I had a short conversation this morning about sharing things. She's been telling me that she, as someone who has never experienced non-monogamy, has been feeling guilty about sexual activities with another man. This guilt gets worse as they progress further, and I am trying to ensure that I am not CAUSING that guilt. I'm not 100% successful, but I'm trying. When I asked her how her relationship was going, she said "this seems to be mostly physical to be honest". That's pretty much in line with everything she's said so far. I remember from even when I met her there's a certain line for her, as someone who identifies as demisexual, where sexual acts like kissing are comfortable with her based on purely physical attraction. As things start moving below the best, her needs for trust and emotional connection increase at a much greater rate than your typical hetero young female.
One thing I DO have to consider is that last night I sent her a flirty/dirty text before bed "don't forget to brush your teeth before bed

" (inside joke, from when he slept in her bed right after I had outercourse with her) and she said "ok.. lol yes I'm staying at his place but we're not doing anything tonight because of mother nature

" then I thought "huh? I wasn't getting at that but okay". This morning she said she had a confession, that she lied about 'mother nature' last night and that her cycle hadn't actually started yet. She told T that so he wouldn't push things sexually, and I guessed that 1) she didn't want him to see her texts and realize she was lying to him and 2) she thinks that I will feel more secure if I know that hard limit exists. I do get that for a young woman to be able to say "nope, can't do it, time of the month" is much easier than explaining why she's not okay with it. And I definitely believe she's uncomfortable with things progressing where genitals are concerned. What we talked about there is that I don't want her to feel like #2 is a problem, but the fact remains that she was dishonest about it at the time.
Galagirl - thank you for the clarification about boundaries. I didn't actually think about them that way. When I said "mentally preparing to break up with her" I meant that I'd preparing myself emotionally, and verbally, to how I'm going to BEST handle that conversation if it needs to happen, and start accepting it may need to happen. I don't plan on breaking up with her right now.
On the counselor thing - I'm actually in counseling right now. My last session with him, Friday the 28th, we spent almost all of the time just getting the story of my relationship out there. It was only my third session or so with him. I definitely know that ruminating/boundaries/control are issues for me, and I'm hoping to address them in therapy.
Believe me, I used to be way worse with control.
On boundaries, so let me check - are these good examples?
1. If you have unprotected sex with another person, I will not have unprotected sex with you until you are both tested.
2. If you are behaving strangely towards me, I am going to point it out immediately.
I agree with you 100%. That's something I've been preaching since I learned it not only in my undergrad, but as a client in therapy. But logic kinda flies out the window when you're under emotional distress, and I think some people (like me) their instinct is to control the stimulus - when it is the response that you can actually control.