You know how to become physically stronger -- just hit the gym and work those muscles. But how do you become mentally stronger?

According to psychotherapist Amy Morin, building your capacity to deal with negativity and adversity isn't about building mental muscle mass; it's about subtracting the harmful habits of thought that waste your energy and make you weaker.

Her book 13 Things Mentally Strong People Don’t Do details exactly the sort of destructive thinking you should cut out of your life if you want to radically increase your resilience. It's based on a post she wrote on the same topic, which went viral. If you're looking to kick some bad mental habits and toughen yourself up, it's a great source of ideas to get you started. Here are a few things you should stop doing for improved mental strength.

1. Feeling sorry for yourself

Your grandmother was right -- there is little benefit in crying over spilled milk. Bad, unfair things sadly happen. Moping about them is just a waste of your energy. "Mentally strong people don't sit around feeling sorry about their circumstances or how others have treated them. Instead, they take responsibility for their role in life and understand that life isn't always easy or fair," Morin writes.

This advice is a bit controversial, however. Other experts have argued that a short, controlled burst of wallowing after a setback is a healthy way to be kind to yourself, process negative emotion, and learn from your experience. But everyone agrees: If feeling sorry for yourself goes on and on with no constructive benefits, it's time to suck it up and take responsibility for moving forward.

2. Avoiding change

If ever there was a losing battle, it's trying to avoid change. Not. Going. To. Happen. Mentally strong people know this. They "welcome positive change and are willing to be flexible. They understand that change is inevitable and believe in their abilities to adapt."

3. Trying to please everyone

Trying to please everyone often means you end up never pleasing yourself. The mentally strong are considerate of others but they don't make this mistake. "They're not afraid to say no or speak up when necessary. They strive to be kind and fair, but can handle other people being upset if they didn't make them happy," writes Morin.

4. Repeating mistakes

You've no doubt heard the oft-repeated (and possibly misattributed) Einstein quote that says "the definition of insanity is doing something over and over again and expecting a different result." The truly resilient take this truth to heart. "They accept responsibility for their behavior and learn from their past mistakes," so that they don't make them again, Morin asserts.

5. Resenting others' success

Envy may be natural but it's also a pointless energy suck. All the thought you're pouring into comparing yourself with others could be put to better use. The mentally strong "don't grow jealous or feel cheated when others surpass them. Instead, they recognize that success comes with hard work, and they are willing to work hard for their own chance at success," she believes.

Of course, that's easier said than done for many of us mere mortals. Luckily, there's practical advice out there on how to talk yourself out of your envy.

6. Fearing alone time

Did you read about that study that found many people would rather give themselves a shock than spend time alone with their thoughts? The mentally tough wouldn't need that electric zing. "They enjoy their own company and aren't dependent on others for companionship and entertainment all the time but instead can be happy alone," Morin insists.

7. Feeling the world owes you something

Those with real resilience expect to work for everything they have. "They weren't born with a mentality that others would take care of them or that the world must give them something. Instead, they look for opportunities based on their own merits."