¿Qué significa It is when we start compromising these essential elements of who we are that the cracks in the foundation of relationship start to show. ?

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This may be a lengthy response I tried my best to keep it to the point. Sorry in advance

Think of a tall building (you),
sometimes it sways back and fourth ( sometimes you get angry, do a bad thing once and awhile)
and the concrete that makes up the foundation can develop small cracks in it. (Your relationship with someone close to you maybe you have a fight now and then)
Now a strong wind comes and the building starts to really sway all the time (you start repeatedly doing bad things you would normally not do, maybe your angry all the time etc.) since the building is constantly swaying the small cracks in the concrete become much larger and a major problem (your bad behavior brings stress on your relationship and the small things that you fought about before become bigger fights )

In other words I think it means-
When we begin to make excuses for our own bad behavior or start to act in a manner that is immoral your relationship with someone becomes strained and the problems in the relationship that were once small become larger problems.

This can be between two people or even between countries

I hope this helped you.