Okay so first of all: congratulations on making it through your interview.
What I think instantly: send an email and put an emphasis on thanking the interviewer personally + explain all the reasons why you want to specifically join this university + ask to keep in touch + if there are some things you could not talk about mention it!!! ==> be subtil though (you will find lots of follow-up email examples on google, adapt it to your situation).
And honestly, it is okay if this did not work out. If this is a fail, ask later on a feedback. Work on other options or reapply next intake but keep in touch and network.