Being Orthodox and an adult single is replete with huuuuge challenges. I don't minimize it and I don't judge people for their actions [at least I try to avoid it as much as my judgmental self can]. The reality is that in most cases [that I have observed], the longer one remains single, the more they fall spiritually, often to the level of dropping observance altogether רח"ל. It is just too much for people to handle the rigors and limitations of religious observance together with singlehood - so they go off .....
What is the eitza? How does one stay religious despite?
In my experience with boys, there are two key factors. One is Talmud Torah. A boy who stays serious about his learning, stays frum. That is a tribute to the power of Torah. The other is having a consistent kesher with a Rebbi whom one respects. One who guards these two areas is most likely to succeed until that magical day when he gives Harriet the ring ["Harriet Mekudeshes Lee"] under the chuppah.
For young women I don't know the answer because my contact with female singles has been relatively limited over the years and the few I have been in contact with remained quite religious. I don't know what did it for them. One thing I know helps is for her to be close to a Rebbetzin [and/or a Rav]. Learning is also a huge boost as it is for the boys but the Beis Medrash framework that exists for boys doesn't exist for girls. And there are plenty of girls who are not serious about their Talmud Torah who remained בנות ישראל כשרות [hard to say the same of boys - especially given the difficulty of a boy transforming into a בת ישראל הכשרה. But today ANYTHING is possible!!]. I don't know what other factors strongly impact a girl's observance but she should daven when she leaves seminary and goes out into the "fake world" that she never forget her faith, heritage and people.
