My own particular flavour (although not unique, of course) is an especially piquant blend of depression and anxiety. As I someone put it recently, “Depression is not being able to care about anything and anxiety is caring too much about everything; having both at the same time is hell.” (I wish I could find who wrote that as it deserves attribution). Those of us so afflicted are buffeted by constant doubt about everything - double guessing every deed, every word - paralysed by both the appalling, deathly lethargy of depression and the terror of the consequences of our own actions. All compounded by the knowledge that we are (I am) utterly worthless, that we probably deserve to feel this way and suffer all that comes with it; part of the reason it is so easy to become isolated is that we know we aren’t fit for human company.
(Even writing that I have to fight the idea that anyone reading it will think how pathetic it is, this blatant attempt to garner pity - or, worse yet, actually pity me, or feel for me; it is made easier by the thought that nobody will probably read it. Welcome down the rabbit hole that almost every single thought leads. Imagine that, constantly, endlessly, not being able to escape that).
About that selfishness; much of it seems to be caused by an over-abundance of empathy. Not just the worry about how my words and actions will be interpreted, but how they will affect others. But it isn’t just the big things; EVERYTHING has to be weighed for its consequences, on whether it is the right / best / optimal / moral thing to do. I am crushed beneath the paving stones of good intentions.
And the more we care about someone, they more important they are to us, the harder it is. The effects are magnified, the potential harm all the greater. The knowledge that I can only hurt them by my actions and, if they are around long enough, finally I will hurt them by my exit, on that day when I ask myself “why not?” and can’t come up with a good enough answer. And this is further exacerbated by the Groucho Effect: why would I want to belong to a club that would admit someone like me? So people who do insist on getting close to us become tarnished by the illness. I mean, what are they thinking? If their judgement is so poor that they want to be around me maybe they deserve it. Which is, of course, just an excuse to chase them away, something else to to beat myself up about. Another failure to cling to, to define myself by.