This is true .. but does not really work with a partner who is very negative ... you lower your boundaries and behave in a positive manner with your partner .. but they keep lashing out in a negative and toxic manner... it is a cruel and hurtful pattern
Recommend you read the book also available in audio format online’ The Power of your subconscious mind’ by Dr Joseph Murphy. Has techniques to help with challenging situations and people. It is more about teaching your own subconscious what you want so it manifests in your reality .Agreed that it can be easier said than done, but it does not mean that one needs to be submissive in order to be positive. It is more about creating a safe space within yourself without compromising on being assertive and protecting your boundaries when required.