... that my female manager flirted with me then denied it? (i'm a girl)

Don't let the bitch fucking gaslight you. Here is a hint, people don't make up being sexually assaulted in their heads. Occasionally there is the case when people fake it but those know it didn't happen. Your entire story sounds 100% realistic up to and including your boss's denial. Here's another hint, boss will NEVER validate your claims. This is an easy opportunity to preserve her professional representation and will do this despite any psychological harm it causes you. Corporate people like this convinced themselves long ago not to give a shit about you, especially when her career and potentially legal ramifications are at stake. You need to trust your reality. Therapy can help but only if you completely understand that it is just for a person to talk to and that you are in no way mentally ill, at least in the sense that you would imagine an assault. You would be very sick and have a completely different life if you were that removed from reality, forget anyone ever said that. She is a sicko and you need to stay away from her. Trust your mind, it is all you have.

This sort of thing is why companies have non-fraternization policies. Someone senior to you made advances, you spurned them, and you pay the price.

Very hard on you, but it was "normal" for her to do this, since it was a case or her or you. It's the sort of thing that can be expected in such situations, although it's cold-hearted and dishonest.

The only thing that confuses me is you saying you were "heartbroken" when she stopped paying attention to you. If her advances were not wanted, why were you emotionally distraught when they stopped? Considering that you had apparently chosen to torpedo a pet project of hers, what did you expect? You say you "stopped it [the flirting] by refusing a joint project". This is the only way you could see to make it stop? Seriously?

Firstly, it sounds certain from your story that the sexual assault is legit.
But more importantly, you should never confront your boss and tell her that she tried to assault you. Of course she will deny it, why else was she giving you slight pushes (indirect hints) because she can back out anytime.
When you confronted her that way she freaked out that you might tarnish her reputation.
Of course it is normal for her to reject anything even though she is wrong in doing so.
You should have waited for her to make a more concrete move so that you could accuse her. But you spoke to soon, next time you'll know better

The feelings went both ways... I resisted her well at first, but it got harder as time went on. Eventually I gazed, smiled, confided, 'brushed against' her, in return. The thing that convinces me I wasn't imagining it all? She wasn't at any point like 'wtf'. There came a moment I'm ashamed of, I stopped just being passive and ignoring/returning her 'advances' and actually gazed deeply at her and smiled. She held the gaze like it was totes normal. An intense moment. I did it cos I'd thought 'why are we just going on glancing at each other and smiling and looking bashfully away repeatedly, as if we both didn't know each other knew what was happening?' It was straight after that that she 'groped' me, if you could call it that. It was only my arm (thank god). But it was the feverish, impassioned way she did it.

The incident broke my heart harder than it has been broken in many, many years. I didn't break it off cos I didn't like it. It was cos I knew it was wrong and I didn't want us to fuck our lives up.

God, that sounds awful. I always hated being harassed at work.

If it never happened, it's perfectly normal. If it did and your manager is distancing herself, that is also normal. What you took from your situation is you were being flirted with, but if there are other plausible explanations then you have no proof that the feelings were ever mutual or that this was the situation. There are lots of reasons to let employees go, from simple numbers to the general attitudes of management above you. Again the important thing is that you don't really know why. You can speculate about it and people online can speculate about it, but there's quite literally no way of knowing. The only thing you can do from here is try to learn something positive from it and push yourself in that direction. Also taking medication is something you might want to journal about, how it makes you feel, what practical effect it's having. Finding the right ones are tricky and sometimes they're ultimately unnecessary, but you should never take unsolicited medical advice from a possibly spurned manager.