Unfortunately, since Mia is back and they've had this many interactions, I feel like all other relationships are kind of doomed. The universe itself is conspiring to get them back together. If Mia weren't so mentally ill they should just marry and make babies with Taesoo. It'd be the best for everyone.
5 years is a long time but to get over what is essentially the love of your life... it can take longer than that. Speaking from experience and anecdotes from people I know.
I really like this series, it reminds me of my "popular period" (loooong gone, I'm a hermit now).