Make other Sims break up

Unlike in the Sims 2, each save file exists in its own, separate universe.

In the Sims 2, time did not pass for sims you weren't playing. However, in the Sims 3, the "Story Progression" means that no matter which household you play, time will pass for all families in the neighborhood.

The one thing that is shared between neighborhoods is the Family bins -- that is to say, both you and your fiancee should have each other's sim in their neighborhood bin. If you then move this sim into the neighborhood (even if you don't change the active household), you will be able to find, meet, seduce, and eventually marry each other's sim.

However, because you have separate save files, marrying in one neighborhood won't simultaneously marry the sims in the second neighborhood.

It is possible to simply play multiple active families in the same neighborhood, but since every household ages at the same rate in The Sims 3, this will probably not be a preferable way to play (not to mention losing wants every time you switch active households).

The Sims 3 is notorious for high CPU and resource usage, especially when you get multiple households and things going. Try opening up the task manage (ctrl+shift+esc) while playing and take a look at the CPU and Memory percentages. If they are pegged at 100% or close to it, that may be the problem.

The game does try to "optimize" by instead of the entire game chugging, only certain processes of it do, like sim behavior and the background world simulation (which is why everyone is tired). Something to try is lower the various settings, or removing households you don't really care about so the background simulation doesn't have to work as hard(backup your save in case this doesn't work!).