I'm with @AmytheDancingBrick It sounds a lot like OP was beetling up the inside lane at her speed (52mph? 58mph? 65mph? 70mph?) and the bus was (correctly) using the slip lane to try to match the speed of the inside lane as closely as possible.
Yes, it's possible that she ended up in blind spot & bus ended up in a sideswipe manoeuvre oblivious of her existence. It's also possible that the bus was paralleling her for 100m or more, unable to accelerate above 60mph due to its legal speed restriction.
At which point the onus would be on OP to jolly well get on with it & accelerate to 70 to open up the space behind for the bus to move into, or to gently ease off the accelerator to allow a safe space to open up in front of her for the bus - which had been alongside her or close enough to alongside for what was probably nearly 200m of sliproad at this point.
Sometimes, being an anxious driver means a lack of positive decision making in a scenario where the end game is pretty flipping obvious. Relying on being the 'right of way car' and not matching your speed to the sliproad speed by adjusting +/- a few mph is not appropriate in this situation.
I very much doubt the bus appeared from nowhere. It certainly seems the OP was relying on it stopping rather than making it clear whether she wanted it to go into the space IN FRONT of her, or accelerating promptly out of the blind spot to create a space BEHIND HER for it.
I feel for you being in shock OP, it's a very real set of physical symptoms, and I hope you can reflect on how to keep you & your family safer once you've got over it. As others have suggested, some advanced driving lessons would really help, as you can do a lot to improve your anticipation in situations like this. 💐