I left the instance before entering the portal needed to transport me to Thunderhead Peaks and now am locked out from getting there. Is there any way to fix this?
You might not want to, but could you just replay the episode?
I thought about that but the only option I see is to cancel the chapter. When I do so I can only resume it again, no option for starting over.
Cancel the chapter, swap to another story completely and then start the living world season.... Does that work? Else you probably need a port to friend (a friend inside Thunderhead Peaks)
Play the Chapter on a different character, then purchase the Portal Scroll might be an option.