I'm still really worried about when turning off, that someone from behind comes up at the side of me as I turn and we crash.
That is a risk we all take while driving I'm afraid. But if that happened, it would be their fault, not yours.
Does it really matter what lane you go in? I'm thinking to avoid crashing it'd be better to just always stay in the left lane no matter where you're turning off, it'd be the least stressful for me too.
If you're somewhere where you're not familiar, that's a good idea. E.g. if you're on a dual carriageway and you think your exit is coming up soon but you're not sure exactly when, it would be silly to go into the right lane to overtake someone in case you can't get back in to the left lane and you end up missing your exit. Personally if I'm somewhere I don't know very well, I stay in the left lane unless the signs direct me otherwise.