In order to define this quote properly, I believe it is absolutely necessary to have a full understanding of the characters throughout the play. Stella, who says towards the end of the play "I couldn't go on believing her story and live with Stanley", has gone through quite a lot between hearing the malicious rumors of her sister at the "Flamingo", and Stanley physically and verbally abusing her. Stella is in a very difficult situation, especially considering she just gave birth to Stanley's child. If she chooses to believe Blanche, who has already deceived her numerous times since her stay in New Orleans, than she loses Stanley and the provider of her and her baby. But what does she come out with? A Manipulative sister who has gone completely insane and could be no good to her and the baby. I wouldn't be surprised if Stella is just so terrified of what became of her sister that leaving Stanley and being alone would drive her to the same level as Blanche.
The best way I know to explain this situation is from personal experience. Almost a very similar story to what we have seen in A Streetcar Named Desire. While I was away at my SUNY Delhi my sister, who was a young single mother, called me telling me she was pregnant again. She was so torn between what decision she would have to make considering she was not to certain about the father to be. To make a long story short my sister committed to her responsibilities and although she doubted highly at first how this man she was with would take the responsibilities, she let herself into believing it was meant to be and perfect. She gave birth to twins in March 2009, and the babies father ran. Under the most severe situations, when you feel almost hopeless, your mind will let you believe whatever helps you get through the day. My sister knowing and thinking clearly at first did not have faith in this man to hold to his word and to man up to what was to come. However, mentally broken down similar to Stella, she let herself become blind to the truth.
Nobody is perfect and we all make mistakes, and we can all let ourselves believe in false truths due to mental instability. The only problem is how long do you really expect everything to seem like a Disney Movie all perfect and everybody getting their "happy ending". If they made a sequel to this play I'm extremely curious just how Stella and Stanley turned out. Maybe she allowed herself to fall so far into this Illusion that it became her reality. I just feel that eventually, as proven in my sisters case, the truth will come out!