This is the problem with many people today (well I should emphasize on the over 30 & single population). A lot of women & men who reach a certain age in life begin to get that "fear" of being alone in life. "Oh my, what if I never find that right person?" There is no need to worry or stress yourself out for the "perfect" person because perfect doesn't exist. A great person does.
When you keep focusing on all these qualities a person is suppose to have you end up not finding them and finding flaws in many of the ones you meet. So by the time you hit that age you find yourself surrounded by all your married or engaged friends. You end up dating and "settling" for the girl or guy who is not the ugly duckling or nerd yet the sincere, loving, kind & great friend who you end up caring deeply for. They will be the one and only you will know why. You know they will be the one that will be loyal & share everything with you as you will with them, the two of you will share great companionship, trust & a bond that no one can break. You love each other in ways no one can understand, that is why the relationship works. It's idealistically perfect.
If you have lived this way & it seems to the "perfect relationship" for you; fine, but what happens when one of you begins to question if marriage will ever come about? The other simply says there will never be one, there is no need to change what is already great. Hmm, questions begin to rise in the female mind, romance & desire are lacking, the yearning for passion teases the minds. The entire "relationship" is put to question "have we settled?"
When you find yourself questioning your own happiness and wondering if there maybe other opportunities for you out there; you have answered your own questions. Many people & I will openly admit (women) will stay in a relationship for financial reasons, parental obligations (but when they're no children involved) it is mainly due to fear of being alone. I understand it is easier said than done but when done so you live a more eased state of mid knowing you are not settling & can be someone's best, YOUR OWN.
No one NEEDS to be in a relationship to not feel or be alone; having great friendships with friends & family & spiritual ones will guide you to a person who will love you how you deserved to be loved, just love yourself enough first to not settle for anything less.