Fortunately when you leave Sumas you'll be driving straight and won't have to back up. Depending on which way you head out of there, you will have back roads (lots of farms to drive by) to drive on which are curvy and single lane each direction. Just take it slowly around the sharp curves and don't worry about what's behind you.
It took me a long time to get past the worry of who all is behind me.
Regarding doing it yourself - that's what I do. Except for two weekend trips with Dirk along, it's been me, myself and I on the trips. We got the rear view/back up camera on the trailer, which has been invaluable. Make sure you have good mirrors. I also have a back up camera on the Tundra which really helps for hitching the trailer. ETI put a strip of orange reflector tape on the ball of the hitch and that really helps in lining it up for hitching. It took a bit to know where that line was in the camera for perfect alignment, but now I can usually do a better job on my own than when Dirk is helping me at home to hitch up before I leave for a trip. I've also impressed a few guys (and women) at campgrounds with getting it lined up without any help from someone waving and/or shouting at you as to which way to turn to line it up. :whistling:
I picked up the trailer in November and drove it home, after taking a nap at the first rest stop past Bellingham. A neighbor put it down the steep driveway for me; I can now do it myself. When I was ready to start using it, I took a two-day driving course through RV Driving School. They use contractors that give lessons for motorhomes (vast majority do that) and travel trailers (not as many). I took them through a fellow in Sequim; they now have someone else doing them part of the year in that area. I thought about the school next to ETI but decided I wanted the two day course instead of a few hours as it was all new to me. they have instructors in different parts of the country so there might be one that will work for you. I found the videos to be worthless as I'm not the type to learn that sort of thing that way. Some are; I'm not. You might be.
Once you get the backing up figured out, find a big parking lot without any traffic and do lots of practice. I take it back out at the beginning of the season to refresh my skills and that really helps.
When you plan your route, if googlemaps says it's, for example, a 5 hour drive, add several hours to that for rest/food/pee/gasoline breaks. Plus you probably won't be driving the speed limit of 70 or whatever it is. In CA you'll have a 55 mph limit for towing. Which is fine by me.
Depending on how many exit/entrance ramps there are on the highway, I tend to drive in the middle lane if there's lots of them as it can be tricky slowing down/speeding up for traffic coming in, especially all those semis. If there aren't very many of them, I'll stay in the far right lane if there are 3 lanes. Everyone else will want to get around you, to put it mildly.
There are websites that will tell you where the rest stops are. I print that information out so I have reference as to when I want to pull over, as my GPS is pretty worthless at telling me even though I have it set up to point them out. The northern OR rest stops are nice with lots of room. Some even have extra space in the back where you can pull over away from most of the semis and take a nap. Farther south they're lots smaller and some times I didn't always have room to pull over for a while. And for the trucks and trailers, it's just a long row where you pull over to the curb on either the right or left side. Not pull through sites. Let's just say you might have to pull off on an exit ramp if you really need to pee and the rest stops are full. And that can be tricky at times too.
When you come through Seattle be sure to stop at the rest stop in the Everett area (north of Seattle), which is the last one for a long time. They separate out the semis (they stay in the truck weight station area) from the RV area, so it's nice if you want to take a long break or a nap. You won't find the next rest stop until you're way past Seattle and down past Olympia. Traffic can get really boogered up around both Seattle, Federal Way and Tacoma; so yeah, pee when you can! ;D
Coming through downtown Seattle you don't want to be in the far right lanes as they eventually get siphoned off for another highway. I get in the next to the far left and that works just fine through downtown. That way you'll be in the proper lane to stay on I5.
Going around Portland, I take the 205 bypass as the bridges in downtown Portland have always majorly freaked me out and even more so pulling the trailer. :nonono:
South of Portland there's this one exit that a lot of truckers use to fill up. I forget the exit, but someone from around there could probably chime in. Many times the far right highway lane will get wadded full of RVs and semis pulling off to get gas. It's not very well planned at the end of the ramp so traffic backs way up, including the shoulder of the highway. Before you get there, don't be in the far right lane if at all possible. It's dangerous. I got gas once there when I was heading north and holy cow, never again!
Have fun with your new trailer! I thoroughly enjoy using ours. It's "me time" as Dirk says, when I take it out.
White knuckles and I made it home just fine. I think in my case, "baptism by fire" actually helped. It gave me confidence I may not have gained if traffic would have been light... maybe. I do remember I burned through enough adrenaline that I was starving when I got home. So watch your blood sugar level, truly. Eat healthy! And good luck. No one is born with a towing gene. 
