What is a good technique for cutting in when painting?
This verges on "opinion based..."


Brush width depends what you are working on - for wall corners, 2" is
probably fine. On our last project, my assistant finally figured out
that using an overly tiny brush on window muntins was slower, not
faster or more precise, but for that job a 1/2-5/8" brush was "about
right."
I go for the long handled angled - how much that is the physical
characteristics .vs. how much that is one of the better quality
brushes I own is possibly debatable.
If the brush needs a haircut, give it a haircut - while this ties
back to brush quality to an extent, any brush can get "flyaway
bristles" - cut them off ruthlessly. For lower end brushes, this can
improve them a lot, practically speaking.
Dipping the brush in water (latex) or thinner (oils) and then
flinging/brushing out the water/thinner will tend to reduce clogging
of the brush "cavity" or reservior for paint, .vs. just dipping it
straight into paint. It's not absolutely required, but it make
through cleaning much easier, and through cleaning is the way to keep
a good quality brush alive for a long time. A good brush costs enough that you should take care of it.
Personally, I thin the paint a bit if it needs it, but figure if it's
quality paint it already has whatever additive package the paint manufacturer
thinks best.


@bibs comment about the death-grip is probably relevant to your pain the last time. The other thing, which does get better, is that this is one of those things where practice really does make perfect (or at least you learn how to make it look more perfect, even if the process isn't quite that perfect) and as non-professional painters most of us simply don't do it enough to build the skill to a high level. As such, just the stress over "doing a good enough job" can make you tense up (and that also makes it harder to paint well, on top of hurting.) Admire an amateur painter's paint job and they will nearly always wander off into to pointing out invisible and virtually invisible flaws which they perceive as if neon-lit with giant arrows pointing at them, even in the face of compliments about how nice it looks.