It's only someone who's an idiot that's going to find it very difficult to apologise when they are in the wrong. Sadly, there are so many stupid people out there now because they don't know how to say sorry. 

It's only someone with ego problem that's going to find it very difficult to apologise for doing something wrong. It's very easy for me to say sorry because I don't like making someone mad at me. 

I don't find it difficult to apologize but I've been in a relationship that my partner finds it so hard to apologize for his mistakes. It was becoming a norm and we fought about it but I decided to end the relationship to have my peace.

It's mostly women in my opinion and experience who find it very difficult to apologise. They used weaponize sex for men in their lives and it's the one and only thing which they hook and bait men.