When a person can not maintain proper friendship how can he/she can maintain permanent relationship?

Depends upon the reasons why a friendship fails. Sometimes it is because there is an attitude or judgment that puts the relationship out of balance. Being conciliatory when necessary, compromising, and having empathy are aspects to cultivate to help propel a long-term relationship or friendship to success.

Those are 2 different things. She may be giving relationship more priority than anything else, hence friendship may suffer. I'd choose a good relationship over friendship any day. I know friends are nor forever.

The same way everyone else does. Lie to one another and delude yourself into believing that what you have is love and not extreme fear of being alone

And yet people do. I know married people who have no other friends. Possible they can maintain friendships and have just decided to let them go in favor of their spouses, I don't know.

I don't think it's possible. but that's just my opinion

You can't.