Standalone established relationship facing troubles

Hi, I hope if anyone can recommend me some standalone books or that can be read as standalone, that feature established relationship hitting a snag and the couple is trying to work through it. How they met can be in the form of flashbacks as a parallel plot, but not more than the present timeline, and not necessary at all. Prefer angst and contemporary in this case.

No BDSM, no Omegaverse, no fantasy genre please. And in this case, no amnesia trope, which is the easiest to qualify and I have read a lot of them. Thanks a lot!