Let me tell you as someone who's followed Twice since the beginning; they do not want to break up and there's an extremely low probability that they will so rest easy on that. As others have mentioned though it's likely that they might reduce activity, "Individual activities will increase, group activities will decrease, but Twice will remain." as Horizonshard said sums it up really well. It could be a good thing as it would allow us to get to know the members even more personally and we'll still see all of them - just not all at once as often as before.
If and when members start to leave the company it will likely be a gradual process, in-house variety like TTT will go first but we'll still get comebacks. (Personal YouTube's might open up instead) Then the comebacks will be fewer and turn into reunions but hopefully by then you will be prepared.
It is sad but as Selfmate said very well nothing lasts forever and we're lucky that Twice is in a position to do what they want (I'm sorry about Kepler though, I didn't know them!), what I would suggest is checking out some other GGs from JYPE, not to replace Twice but just to fill the hole a bit and maybe you'll grow to love them in time.
For me what I do and have always done is channel my love into the members as people, that doesn't make it less sad when I see less of them but it allows me to be simultaneously happy for them having the opportunity to do what they want even if I don't get to be around for it. TLDR; As cheesy as it sounds I genuinely want them to be happy more than anything else, god knows they deserve it after working so hard all this time and even saving my life once.
On that note we should remember that just because we don't see them together doesn't mean they aren't, they'll still be a family with the means to see each other as often as they'd like.