I eventually got over it, and as I got more and more karma, it didn't hurt so much when I lost a point. But being karmassassinated was enough to throw all of that into doubt again

Get a few more (thousand?) karma and you may find getting karmassassinated doesn't hurt much any more either. I get karmassassinated about once a fortnight (frequency memory subject to all sorts of salience biases and utterly unreliable - it happens quite a lot though) and it doesn't bother me all that much.

These days I find that getting the last 50 comments downvoted is a lot less emotionally burdensome than getting just one comment that I actually personally value downvoted in the absence of any other comments. The former just means someone (or several someones) don't like me. Who cares? Chances are they are not people I respect, given that I am a lot less likely to offend people when I respect them. On the other hand if most of my comments have been upvoted but one specific comment that I consider valuable gets multiple downvotes it indicates something of a judgement from the community and is really damn annoying. On the plus side it can be enough to make me lose interest in lesswrong for a few weeks and so gives me a massive productivity boost!

When I was a lurker, the biggest barrier to me saying hi was a tremendous fear of being downvoted.

I believe you. That fear is a nuisance (to us if it keeps people silent and to those who are limited by it). If only we could give all lurkers rejection therapy to make them immune to this sort of thing!