Firstly, next time you are in a long lasting build or decorating play, you need to save often during the process.
Keep an eye on the memory used by the game. I have Windows 10 and watch the memory in the Resource Monitor that I launch from Task Manager, performance page.
Once the memory used gets up above 3,000,000 I know I'm going to be in dangerous territory if I continue to build or decorate and save. The actual number varies depending on lots of factors. But when it gets much over this number I've quoted, then I always close down the game and relaunch before continuing. This clears the memory.
If, however, I've not taken my own advice and gone beyond the game's limit, there are a variety of things to try. One is to find a bare patch of grass or something to focus on. Just get away from the lot. Moving to some other place in the world might get the memory used down enough to be able to save.
If this doesn't work, and I don't care about losing wishes and opportunities, I will activate a small household. Usually with a single townie sim living in a tiny house. This has often saved me when the memory used is too much for the game to cope.
But sometimes all you can do is shut down and leave the scene of the disaster for a while. Eventually you will be in the mood to go back to it and try again, this time saving often and shutting down occasionally to release memory. I've had to do this a few times.