how many people were there on that monday? Sounds like you realize the issue was not the other bike passing and more of a mental thing. Next time, keep focused on your actions on the bike and your secondary concern should be the passing rider....their primary concern should be making a safe pass. If you start focusing on the other rider; whether it be an engine noise behind you or someone coming past, you are going to start screwing up because you just went from 100% focus on your line, turn in speed, apex and all that good stuff to 75%. Cant really speak on the exact situation you went through, but inside passes in general can be a bit ratteling if it happens as or after you turn in. Just know that it takes a tiny adjustment to make room, no need to stand the bike up completely, turn in really late or grab the brakes. More than likely you could have ran your normal line there and been just fine. If they took your line away that is a different story. The only way to get through the issue is to gain more exposure to the situation....that can mean both track time(in novice) AND getting passed(in int.). Stay in I if you are comfortable with your lines and think you will be able to hold them if you can hear a big V twin rumbling behind you. If you think you learned from the situation and would hold it the next time I'd stay in I. If you are just getting comfortable on the track and your lines; bump down, nothing negative about it...there is a lot going on while out there; don't use up your concentration thinking about someone making a pass.