I see some threads on here about going to the mikvah. What is the thought behind that? My understanding is that there's no halachik significance of a man's tevilah b'zman hazeh, except for a minhag to go before Yom Kippur and/or Rosh Hashanah.

In a nutshell, although Tevilas Ezra was rescinded, because it was too difficult for most people, it was only rescinded to the extent that it's no longer a requirement. It is, nevertheless, meritorious to be tovel when feasible. Indeed, the Rambam writes that he was careful about this his entire life.

Aside from that, many people find that going to Mikvah (when feasible) is a very helpful tool to avoid focusing too much on sexual matters - including permissible relations with one's wife (precisely why Ezra made the Takanah in the first place).

There are a lot kabbalistic inyonim involved as well. For example טבילה are the same letters as הביטול signifying the putting on self aside for his creator (as is hinted by the fact that one is unclothed and has no physical importance for himself)

Also one is meant to accept on himself hashem as his king while toiveling and commit to keeping all his commandments.

In short when done right it can be a very deep experience besides for the basic (important) halachik side to it.

It definitely helps but isn't a replacement for teshuva and is definitely not a quick fix.

Lchaim velivrocha

That's interesting. Do you know where the Rambam says that?

It's a well known Rambam, but I don't know offhand where it is. It is not in the Yad Hachazakah.