Welcome to the DhO, Tim.
I like your descriptions, a year and a half of samatha practice would have been, at the very least, a solid grounding to build from.
After a while of this meditation I was concentrating much more deeply than usual. I began to perceive a pile of binders and a shirt on the floor as a bright blue shark. At first I noticed I was doing this, but then I seemed not to be able to perceive it as a trick of the mind, and I experienced terror. My throat began to seize up very tense and I felt like I might vomit. It was very painful. Then the shark faded back to reality, and I experienced another wave of terror which seemed to be trying to find a thought to latch onto.
6th ñana, Knowledge of Fear.
This experience then faded into what I labeled as equanimity. My mind was calm and rational, and easily dealt with thoughts. I decided to try to gain stream entry from here by thinking "what am I missing?" and turning equanimity to the thought. After a few thoughts down this path, I am not entirely sure what they were, I looked at the concept of "I" expressed in the thought. As I did this, my attention seemed to turn back into my head, the thought dissolved, and my consciousness seemed to bulge outward, until "I" was looking fully out of my own eyes. That's the best way I can describe it. I perceived my body as part of my environment, with no separation between the two. This faded back into equnimity, and several minutes later my timer went off. I sat for ten more minutes then got up.
The part I've put in bold is
very interesting, it sounds like you've did exactly what needs to be done while in Equanimity to get Path and it's exactly what I'd recommend to anyone at this stage. The description sounds like you may well have gotten a fruition, did you notice a discontinuity in your experience at any point?
Without more details it's difficult to confirm or deny that you've gotten stream entry, but the descriptions you give sound like the words of someone who's been through it. Even if it wasn't stream entry, the fact is that you know it's possible and that it's well within reach with some accurate attention to this sensate experience.
As for getting back into stream entry again, this has not happened, unless I simply missed it.
I know you probably know what you're talking about but I just wanted to highlight this line. You don't get "back into stream entry", it's a one-time thing, you get Path and go into what's called Review where you'll cycle up and down the ñanas from A&P to Fruition naturally until the next cycle of insight begins for 2nd path. Fruitions are funny, they happen naturally after 1st Path but it's not uncommon to miss them and I know what you mean about frustration when aiming for them at first. If anything, this makes it harder 'cause you're expecting them to happen when they actually happen right when you don't expect it! Take it easy, don't beat yourself up and just maintain your practice, maybe post a practice thread and describe what goes on in your sits, this was it'll be much easier to offer more appropriate advice.
Give it a bit of time 'cause the change that occurs after 1st path is permanent and you'll notice whether or not it changes, subsides or stays as it is. If you find yourself back to "normal" then so be it, you just get back on the cushion and get back to work but I think there's a strong possibility that you've gotten stream entry.
T