A few questions to think about before my suggestions to add to the mix:
1. How comfortable are you bicycling in your local area?, with or without traffic?
2. How comfortable are you bicycling in an unknown area? If uncomfortable, are there things that could make it more comfortable?
3. How much touring have you done? Other than traffic, would it help to have done more?

Depending on the sorts of answers, I'd go slightly different ways:
= If you aren't particularly comfortable riding in your local area. Then I'd seek out ways to build more experience and comfort. For example, local clubs and groups often have regular rides to join. You can do more riding to frequent destinations. You can experiment with mirrors to see if that helps in keeping track. You can gain confidence in riding position relative to the road, etc.
= If you are pretty comfortable with the local riding, but see a bigger concern with those "unknown" places, then I'd approach it a few different ways:
* First an acknowledgement that even with riding a fair amount with commuting/touring, not owning a car, etc. There will still occasionally be roads I come across that take some more concentration or I'm not the most comfortable with. However, my approach is to (1) decrease the likelihood of getting onto those roads and (2) develop techniques for adjusting if they are really bad.
* As far as avoiding, it comes down to getting better at route selection. There are three general techniques I'll use that significantly decrease my probabilities of being on poor routes: (1) use some curated maps/routes, e.g. adventure cycling maps or similar maps put together for cyclists; while there are still occasional busy places, these can significantly reduce probabilities of ending on a busy no-shoulder type route (2) reading others experiences, e.g. journals and learning routes and others from them and (3) going to a place like "strava heat map" to look up where the local cyclists seem to be riding - particularly entering/leaving larger metro area.
* As far as reacting, I've generally have some maps and developed some past experience with picking alternatives.
= If adding touring on top of these other things adds to the concern, then I'd look to pick up some touring experiences with shorter tours and/or tours in the local area where other aspects are familiar but the touring is the new piece.

Overall, depending on initial questions I'd focus on slightly different areas: riding skills, route-finding skills or touring skills.

Everyone is different, but I don't think a little jitter/concern/anxiety is uncommon. Prior to my first tours I was occasionally a bit more concerned. I'd take that concern and try to channel into preparation type areas (checklists or learning from others journals or cataloging possibilities along the way). Mine were probably less about riding/traffic and more on making sure I had everything organized and the trip didn't hit other snags. However, I also found that once I was on the tour itself, focus very quickly shifts - since I'm now experiencing things and adjusting overall.