Is it easier to paint straight lines with a smaller brush

I've tried a bunch of tools for cutting in corners/edges (the worst part of the job, IMO), and always end up going back to using a paintbrush. There's not enough of a texture difference to notice it unless you have your face ON the wall (once it's dry), and it's much easier to deal with.

My family helps each other with painting, and my sister has textured walls. In that case we did use foam rollers to cut in, just for the better coverage in the texture. As long as you have plain, flat, drywall, brushes are the way to go IMO.

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.