The relationship will grow more and more miserable because they refuse to acknowledge how you feel or do anyhthing about it.
The hardest words for a person to say may be "I'm sorry", but when they mean it, it can also be the most therapeutic as well. When somebody you know is never able to say sorry for hurting you or disrespecting you, you deserve a proper apology not just for yourself, but for the wellbeing of your relationship as well. And if the person can't give it to you, then wanting to cut ties with them might be what's better for you because of these reasons.
People make mistakes and it's perfectly okay to do so. But when a person is hurting you in the same way over and over again, refusing to admit what they've done or worse, pretending like they haven't done anything wrong, you stop wanting to trust them. You hate feeling so hurt while they seem completely unaffected by what's going on.
Being unable to admit the full extent of their mistakes and failing to give you an apology that matches that mistake shows how emotionally weak they are. They will never take responsibility for anything in the relationship and will call you "over-emotional" or "too sensitive" or say "you don't have a sense of humor" because of what you feel.
They may lie, they may get manipulative, and they may even get toxic just to make you take the blame for everything that goes wrong in the relationship. It can make you feel excruciatingly alone in dealing with your feelings while your partner doesn't bother helping you through it. They hurt you once, and then they hurt you more before you can get over the first time.
When they are too self-centered or too selfish to see past their own world, they will never be able to accept your point of view on anything in the relationship. Seeing that they cannot accept how badly they have hurt you can make you feel like your thoughts don't matter, which is something you should never be made to feel in any kind of relationship.
5. They don't deserve you if they keep dismissing your feelings
No matter which relationship it is, you don't have to be the only one apologizing for everything. Having your feelings and opinions dismissed over and over again can make you feel small and desperate because they've hijacked the entire relationship. When you are willing to accept the full extent of your mistakes and apologize appropriately, you deserve the same treatment from the other person as well.
Giving you a half-hearted apology or meaningless promise that doesn't change anything in the relationship could be their way trapping you in the relationship. They make you believe that they will change and then eventually go on to hurt you in the same way; neither will they be ready to make things right in the relationship and neither will they let you off the hook.
A true apology where they accept all that they have done can change your relationship and bring you closer. It's possible to heal from the pain that they have caused, but you can't do it if they don't give you the chance to. If they are not willing to move on, they won't let you do it either. And holding on to that kind of negativity can be dangerous for any relationship, and walking away can help you start over with a clean slate.