I think this is something literally every writer goes through. Like you come up with an idea and you either write it down or expand it and get it all finished and then sometime later you come up with a different idea, and it involves something new or different and you're convinced that it is better but you can't get it to fit into what you have because that stuff is already done, so you get all frustrated and consider reworking everything, or just moving forward with what you have, but you know deep down that the new thing you came up with is good and it's unique and most importantly you really like it.
So... What do you do?
Well, not really anything. The main options you have are giving in and reworking everything for this new idea, or scraping the new idea and just being happy with what you have.
Alternatively what if you can have the best of both worlds. Instead of figuring out how to shove this new addition into your plot now, and rework everything why not save it for a later arc. Maybe the next segment of your story could benefit to use it instead of another part.
The other thing you need to consider also though is if more is better. It's easy to think a new idea is the best because it's new, but is it really better?
Why not make two drafts. One as they are now and one with the new cool idea and see what turns out better by having a friend or something read it.
You'd be surprised how many 'cool' or 'new' ideas aren't actually as good as what you had before, and it's just your brain tricking you.
Anyway, probably didn't help, I sort of just rambled, cause I'm stuck at work lol.
Good luck.