Guys I'm not understanding like what he means that boundaries are weakening

I imagine it as them together as one but then it shattered somehow in the past creating what we have now. Multipersonality schizo mugo

There are 3 personalities in one body, and asshole Mugo as we can see is the one who appears the most, while his personality is dominant he's still able to communicate with 2 others seeing them in mirrors and reflections but he has mind of his own, same as them. Since he's the one who's majorly present, probably means he was capable to suppress the other two easier and more often. For example Sir. Mugo barely appeared because asshole Mugo just take over whenever he wants.
Thing with multiple personalities disorder is that each person is like totally separate person with their own consciousness and memories. Many people who suffered from it said other ones were communicating and plotting things, even talking different languages tgat OG person don't know and despite sharing same body they don't share mind.
But in this chapter asshole Mugo was left with some memories of monster Mugo and couldn't take control as dominant personality that easy, which he described as boundaries weakening as if the boundaries between 3 personalities disappear they'll probably 'merge' together which further will result in 2 of them disappearing. And he obviously wants to be the one who stays. Scary thing with this disorder is that OG personality can be lost for good.

Sorry for yapping this much, I always exaggerate with explaining

This is exactly what I think will happen but I’m also kindaaaaa hoping it doesn’t

Same! Because if at the end only one needs to stay, at least let's wait until we get a backstory of how Mugo became like this! It's to early to lose 2 of the remaining.
Also, something is telling me that maybe asshole Mugo is actually OG or at least he isn't as bad as we think. Monster Mugo is probably way more sinister and since Sir Mugo is kinda still mysterious, he may as well not be so nice (plus asshole Mugo was talking in reflection with one of the 2 remaining personalities that appeared to be quite evil and less silly behaved than monster Mugo, so what if it's Sir Mugo?)