Easily it was my traumatic brain injury I suffered in a motorcycle accident.
The best way to put it is you stop feeling the same way about stuff.
People, places, food, hobbies, memories. The last one of which isn't as bad as it sounds, because despite you thinking otherwise memory is always a fickle thing and prone to confabulation, mutability and altered states of engagement with another.
Hence why when you hear two old friends reminisce a lot of the times one or both in the span of an hour will say; "Hold on a tick, I thought such and such met us afterwards!?" >>> "Oh yeah! No, you're right!"
In truth there's a good chance both of them are wrong, and the memory has been sufficiently altered simply by continued correspondence of the fact, and they've run enough experiments into false memory phenomena to realise just how fucking bad memory is of keeping 'facts' together.
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But a traumatic brain injury? You blatantly know you're not you anymore. It's not merely a confabulation of the things you once were or did. You start to lose friends because, well, you're not like who you were. Either you or they can no longer connect and eventually relationships fall apart.
You're a different person. In my case, I had to relearn to walk, relearn how to express myself, deal with the fact that you can't connect with people you thought of as once friends. Everything feels off and reality will never be quite the same thing again.
To put it plainly, it's like rebuildng a new reality and systems of engaging with it.
This is why many people who suffer a TBI, they get so easily frustrated in situations that seem so very simple, or trying to fill out documents, or negotiate with former friends or family who once had a certain level of 'tolerable annoyance' factor that a person may have weathered but now no longer. Hence, anger and depression come with it like an 18 wheeler. It was my injury that made me want to study psychology, however. Get in deep about how the brain works.