How do I deal with unexpected situations?

Someone sat on my seat at the train (it’s a reservation/booking only train) because they said someone sat on theirs.

I literally blanked out, I didn’t know what to say and it never crossed my mind to asked them to at least ask the people who were sitting on their seat to move or check their tickets.

I was dumbfounded because I’ve never experienced something like that before especially I’ve been on this train multiple times now. Thankfully there’s a conductor that check on our tickets and I was able to sit at my reserved seat.

(They were in the wrong carriage)

Now I feel anger at myself and at that situation because I know I could’ve said something. Now I want to be able to handle similar situations like these in the future.

I’m trying to be more assertive or confrontational but it’s hard when I feel like I have to be angry or express anger rather than be polite.

To add, I’ve been feeling isolated and there are days where I don’t talk idk why something’s wrong with me and when I speak, it’s always a low voice…any advice?