Anyone got a cure for Restartitis?

Hmm have you considered making your own save ? Renovating the lots, making over all premades or, if you don't like premades, putting your own sims instead, like those you no longer play; you would probably enjoy seeing them around as NPCs.

With the work invested in that save, you'd probably hesitate before starting over (I know I do), adding to it instead when you're bored. The roots come from the fact that everything you see when playing is yours and have your stamp.

Perhaps they're too planned out? Not to say that part of the game isn't fun or doesn't have its function but sometimes if my sims are too planned they don't live up to my own hype or I'm already done with the story I've made. I've kinda mentally already experienced it once so I'm not into their story in its actuality if that makes sense. I do much better with a basic idea or no idea. I'll think of a sim like "I want a mischievous and romantic secret agent who switches career paths after discovering something"

That's all I knew about my Sim Ava, for example, and she's had a very long story. She switched to mad scientist after being abducted. I didn't what discovery would make me choose to leave the secret agent path. It seemed fitting. She's sought to be the most powerful and persuasive sim since and her story is fun. She started out babysitting other sims toddlers and kids (my way of aging up neighborhood story baby explosion) for a hefty price. She conned a lot of sims in the beginning.

So maybe it's okay to sometimes just have a loose idea and then let the story roll from there. After "knowing" my sims awhile I will choose the world that fits them best and then build a home. In the beginning they're all always in starter homes for me if they're newly created in cas.

I used to have this "problem", but at some point I just decided to go with it. But instead of starting new saves, when I get the bug to start over, I just start a new household in an existing save. I have 10 saves and usually between 90-100 households I actively play across those saves. That gives me enough variety to keep it interesting. If I feel I'm finished with a household for the time being I "retire" it, which means I mark it as unplayed. I keep it in my favourites though and make-over Sims as they age up and quite frequently I go back to the household at some point. For example, last week I played with the offspring of a Sim I created six years ago and had so much fun with him starting a family of his own. I enjoy playing this way a lot. Now I'm going to jump back into my occults save to continue playing with my new werewolf!

You might find some tips in this recent thread.

What Makes You Restart?
https://forums.thesims.com/en_US/discussion/999889/what-makes-you-re-start/p1

I've been spending the last week(?) setting up a new save file: all the premades are madeover or deleted (bye Charm family, you will not be missed), lots replaced or rebuilt (more the former, I'm not a great builder) and degrees, reward traits and skills added to sims as needed. As well as setting up all my mod added content and skills.

With the amount of effort this is taking this is my final time doing this. Once I'm done I'll make a backup of this save if I ever want to restart and then I'm going to play, adding new households and worlds as they come.

Maybe the CAS, building and planning part is what's fun for you and there is nothing wrong with that if so :)