A lot of people here will be buried with their Highway Code.
I know what you should do when joining a motorway but I also know lots of drivers do things they shouldn't and you have to keep an eye out for them especially if they're driving lorries and buses and you are in a small car.
That's why I agree with PP who've suggested moving safely over to the middle lane after passing an off-ramp (mirror, signal, glance over the right shoulder, manoeuvre) which will give you plenty of time before the slip for drivers who want to join and then I'd get back over if it was convenient. But if traffic was moving more slowly than I wanted to travel I would continue driving in the middle at the same rate as others but within the speed limit until there was a decent stretch on the left.
If I wasn't paying attention; none of us does all the time; and there wasn't space I might take my foot off the accelerator anticipating that the other car might barge in front of me. It would depend on the situation but I would try not to use my brakes unnecessarily on a motorway to avoid alarming people behind me and causing bunching or worse, a pile up, from which I drove away scot-free shaking my head at people's terrible driving in my rear view mirror.
I know it is your right of way but that's no comfort when you're in the graveyard, though they could put it on your headstone.
OP we have all had scary experiences when driving. Sometimes it's something we have done and sometimes it's the fault of others and sometimes it is road rage which is sobering when you realise: "This man actually wanted to kill me because I was driving a nicer car than him and was in front."
Normal people like you and me think about it and sometimes we cry. After one incident I drove home in less than 10 minutes calmly, I think. I got indoors and my heart started beating like a bunny rabbit. You know those Bugs Bunny cartoons where his heart is punching out of his chest? It was almost like that. It didn't help that at that moment the phone rang and when I explained my near death experience the person on the other end said: "I wouldn't have done that. DH always says..." It was not the time for a lecture. That was 30 years ago and I hadn't done anything wrong except not realising I was in front of a nutter who quite possibly has caused death and disability and may be dead himself now.
So I'd follow the advice of people who tell you that they anticipate all other road users to be twats and take avoiding action where possible over the advice of people who tell you what the Highway Code says.
Oh, the other thing I've learned is you have to stop apologising for yourself. You have a driving licence and you have the right to be on the road. You should literally steer of nutters but should try not to be so intimidated by them that you make a stupid swerve that endangers some other completely innocent person.
Just get out of their way when it's safe and fgs stop saying you're a nervous driver. If you are, get extra practice or stop doing it.