7 expensive mistakes couples make during divorce, according to a lawyer

• Sometimes, the qualities that make you want to divorce a spouse make the process lengthier.
• A divorce lawyer shared some of the most common financial mistakes clients make.

No matter how determined you are to prevent getting divorced, you can't control what your spouse wants — or how they'll behave throughout the divorce process. Sometimes, the characteristics in your partner that push you to file for divorce are the same ones that can make the process nasty, drawn-out, and costly, according to Lois M. Brenner, a divorce lawyer in New York.

"Very often, people come to me who are married to a sociopath or a narcissist and they say that they were love-bombed and they had no idea who this person was," Brenner told Business Insider.

Brenner, who took a break from her law career to become a physician assistant, uses her knowledge of personality disorders to help clients protect themselves during their divorce proceedings.

Brenner shared some of the biggest mistakes people make throughout the divorce process, including before they decide to part ways.

They don't talk about money before marriage

In general, many experts (including divorce lawyers) emphasize how important it is to talk about your finances before you get married.

"When you're dating someone, they're probably not going to tell you about their debts, so that's a little hard to pick up," Brenner said. Unless you have incredible intuition, you won't know how much of a spender or saver your partner is unless you talk about it.

If there's a clear imbalance in earnings or assets, she said it's "probably a good idea" to get a prenuptial agreement together. Plus, the process should give you a better understanding of what each of you brings to the table, such as debt that could complicate a divorce.

They're not involved enough in the prenup

Just getting a prenup doesn't save you from financial problems later on.

"I can tell when I look at a prenup that's about 54 pages long that they had taken away a substantial number of the rights that a married person has," she said. "Often, a lawyer will just make sure that the other person doesn't get anything."

But in some cases, like when moms are out of the workforce while raising kids, getting nothing out of the prenup sets a person up for a disadvantage should they get divorced.

Her advice to couples is to discuss what they want in the prenup before going to their lawyers so that they're fully knowledgeable about the agreement. If one person refuses to talk about the details, consider that a red flag, she said.

They don't know their rights in the mediation process

Brenner said some people are too generous in the mediation process because mediators can't give legal advice or take sides. As a result, a partner might give away more assets than they realize.

Brenner said that when hired as a mediator, she'll sometimes recommend that clients see a separate lawyer to help them through the process "so that the other spouse doesn't walk all over them."

They make big decisions without talking to their lawyer

When a client considers divorce, Brenner usually starts with a thorough consultation.

"You really need to have a sense of what is possible in a divorce," Brenner said, such as knowing a realistic range for spousal support if you have kids.

In-depth discussions are important because they also prevent you from making mistakes that can cost you rights down the line, like moving out or not letting your spouse see your children without a court order.

They overlook postnuptial agreements

Brenner said postnuptial agreements can have some benefits, especially if you feel like you might want to get divorced but aren't ready to yet.

She said that when a client appears to be the victim of emotional abuse or manipulation, she might suggest a postnup "so that they can get a decent agreement while they have the leverage of the other person wanting to stay married."

Then, once you're sure about divorce, the agreement can make the process easier and get you a better deal.

They draw out the divorce on purpose

Brenner said that she frequently sees clients (or their spouses) draw out divorce for the sake of punishing the other person.

"They're angry that the other spouse doesn't want to be married to them, so they will do whatever they can to take revenge," she said. "Part of that consists of drawing out the process so that it costs a lot of money and takes a lot of time — it is really excruciating."

They still don't understand their spouse's behavior

While you can't always know you're dating a narcissist or dark empath while in the thick of it, you owe it to yourself to learn more about your spouse throughout the divorce process, Brenner said.

"If you know what you're dealing with, it's easier not to get taken advantage of," she said. Whether your soon-to-be-ex-spouse has a diagnosed personality disorder or just displays narcissistic traits, she recommends reading books and going to therapy to navigate spiteful or unpredictable behaviors.

The sooner you start, the better. "A lot of people tell me, 'Oh, I read this book about narcissism and I realized what was going on,'" she said. "Being educated about these things is really critical."