@hiraetheanine asked in the comments of a post for some pointers on Final Fusion and I felt it'd be better to make a post here; @hiiragi7 @subsystems @theorionissystem (<- i forget what late stage recovery you guys are at) tagging in case yall have two cents to throw in
Largely I did want to say first and foremost two things.
1) this is me just writing off the top of my head a few things that come to mind so I'm not putting an extreme amount of thought and/or formality into this
2) recovery, fusion, DID and all that is always a very individualistic experience and recovery journey so while there are things I say here that can be helpful, this is based on how it worked for me and may not be the same for you (could even be entirely different!); its 100% best to just kind of follow your experiences where they take you ideally with a therapist to help
With that out of the way, I think when it comes to seeking final fusion - or as I prefer to call it "full integration / a state of being fully integrated / full fusion" since I really dislike the connotation of the word "final" - it's kind of best to not really SEEK it out too much.
By that I mean that I found that whenever I "tried to fuse and integrate because I want final fusion" I found that I actually made slower progress and arguably - at some points - reverse progress in the sense I became more dissociative and had more conflicts with parts. Maybe its the Buddhist in me, but I find that fusion is one of those things that is actually deceivingly simple and natural and as a result, actively chasing or seeking it out tends to make it more complicated as you start to have to deal with what you Think Fusion Is rather than just simply experiencing it.
I think the best way to kind of progress with integration and fusion is largely to just kind of throw out a lot of notions of late stage recovery other than a "that would be a nice result" and just focus on learning about yourselves, building relationships with yourselves, and developing hobbies and interests that parts can engage with together. The level of which parts are connected, integrated, and/or fused tends to just kind of come along the journey of building a very strong, very firm, and supportive relationship with your parts.
I think a lot of people think of final fusion as this phenomenon where parts disappear and a new person is formed - and while that might be the way it is for some people - for me its a lot more like slowly becoming co-con with another part so much to the point you don't have to explicitly internally talk to be on the same page and experience things together.
In that sense, I think focusing too much on what parts are doing what and where they are in the stage of fusion can really kind of "backtrack" the fusion (Which! Is not bad! Sometimes fusions don't work the best and don't work well for the situation and its better to say 'yeah no this isn't working' and backtrack. I 100% support undoing fusions if they don't work and we've done is SEVERAL times on the way to where we are now; undoing a fusion also doesn't mean final fusion is impossible; it just means those two as they are are likely not the best combo).
It's honestly really just as simple as just kind of learning to love, care, support, share, and enjoy life with your parts and the more universal that support, love, and care is throughout the system, the more integrated and possibly fused you'll find yourselves becoming.
It really wasn't this sudden night or day phenomenon for me as much as it was a realization that I really didn't cling or see myself much as any specific part and that most of the time, the days I did identify as primarily a specific part (let alone ONLY as a specific part) became the odd days whereas me just being "me and all the parts around idk" became the norm.
I dunno, I guess its kind of a non answer but I just kind of used this as an open mic to ramble. I'd love to answer some specific questions since its honestly a large topic and theres no way I could reasonably come up with ever little aspect of how this happened and things that helped / caused issue in it all