The unemployment rate fell below 4 percent in July, the lowest it's been in nearly two decades. The job market is booming and the economy seems strong. But millions of Americans are still struggling to make ends meet.

That's in large part because wages are not keeping up as day-to-day costs continue to soar, according to Alissa Quart, executive director of the Economic Hardship Reporting Project and author of the recently released book, "Squeezed: Why Our Families Can't Afford America."

"Stop blaming yourself and start blaming the system, or start blaming the deeper causes of your economic fragility and instability," Quart tells CNBC Make It. "There are forces that are constructed against you, everything from your taxes to whether you can have job security."

Middle class life is now 30 percent more expensive than it was 20 years ago, Quart writes — and Americans are feeling the squeeze. Salaries just don't go as far as they did to cover the necessities. Pew Research recently found that the average paycheck has the same purchasing power it did 40 years ago.

Families must also deal with the soaring cost of college, housing and child care. The cost of public universities doubled between 1996 and 2016 and housing prices in popular cities has quadrupled, Quart says. Many families are putting up to 30 percent of their annual earnings toward child care.

"The cost of having kids can seem like Eric Carle's 'The Very Hungry Caterpillar': just like the caterpillar in the classic children's book, your child eats up every dollar you earn," she writes.

Desperation has forced many to take on more work to make ends meet. About four in 10 Americans hold some kind of second job, according to a July Bankrate survey of over 1,000 Americans. Among millennials, that rate is even higher, with over half (51 percent) engaged in some type of side hustle.

And even that may not be enough. Americans born in the 1940s had a 92 percent chance at making more money than their parents. Millennials born in the 1980s only have about a 50 percent chance of doing the same, according to a 2016 study by the Equality of Opportunity Project.

They have less help from safety nets and their employers, too. For example, most young Americans no longer have access to the pensions a lot of their parents and grandparents relied on, and health care costs are often unpredictable and also increasingly expensive, Quart says.

"There used to be a steady, almost boring quality to being middle class," Quart says. "Now it implies insecurity and instability."

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