That's actually a pretty heavily used route, too. Don't know if you've done it at rush hour or not.
There has to be a way to learn how to do this... It can be a challenge for anyone to tell where cars are at night on the freeway, but I don't care how strong your faith is, "just pray alot" isn't a good driving tactic! I worry about you. Are there better mirrors you could install? LSAggie mentioned mirrors once.
I've not done the Yelm Highway route during rush hour, except for portions of it. The thing is, even though it's busy, really you just get on and drive. And there aren't people trying to get on and off except by regular old right and left turns. There are a few places where there are two lanes, but you don't really have to do a lot of merging in and out, not on the parts between Lacey and here. But it's still a long way out of the way.
My biggest barrier is probably still mental, and that's that I'm afraid of getting in the way of other drivers by doing everything at a snail's pace. I can get in OK if I'm really careful...but any kind of tiny hold up can make everyone mad and cause other problems. I guess I'll at least get better at it, most likely, though I doubt it'll ever be something I'm comfortable with. Just the way it is.
I've come a long way, though.