Whether you're tempted to skip the gym because you don't feel like getting off the couch or you're questioning whether you were cut out to be an entrepreneur, it isn't easy to summon the mental strength you need to reach your goals.

But before you blame your less than stellar performance on your lack of God-given willpower, consider this; it only takes a few minutes a day to start building the mental muscle you need to reach your greatest potential.

Building mental strength is similar to building physical strength. Doing 50 push ups a day would only take a few minutes of your time. But if you did it every single day, you'd develop some impressive upper body strength over time.

The same can be said with your mental muscle. It only takes a few minutes each day to train your brain to think differently. With consistent exercise, you'll build the mental strength you need to manage your emotions and behave productively.

While there are many mental muscle building exercises out there, these three are quick and simple ways to grow mentally stronger in fewer than five minutes a day:

1. Act "as if."

It's a natural tendency to act in accordance with our feelings. Consequently, it can be tempting to wait until you feel different to make a change.

But putting off applying for a promotion until you feel confident or declaring you'll get your finances in order when you feel less overwhelmed aren't good strategies.

Instead, studies show you should behave like the person you want to become. Take action first and you'll change the way you think and the way you feel.

Here's an example. When you feel sad you're likely to hunch your shoulders, avert eye contact, and participate less in conversations. Those behaviors keep you in a depressive state.

But if you smiled, put your shoulders back, and started some friendly conversation, you'll feel an instant boost in your mood.

So don't expect your emotions to magically change. Take action and make it happen. When you're feeling insecure, ask yourself, how can I act confident? Acting confident will reduce your self-doubt. And research shows acting confident even increases other people's confidence in you.

So the next time you're feeling stuck, ask yourself what would a mentally strong person do? Then, act as if you feel strong already.

2. Practice mindfulness.

Rehashing something that happened yesterday or predicting horrible things could happen next week will hold you back. The only time you can change your behavior is right now, so it's important to be able to focus on the here-and-now.

A multitude of studies have found mindfulness provides physical and psychological benefits. Reduced stress and a more compassionate inner dialogue are among the many ways mindfulness can help you build mental strength.

So take a minute to pay attention to what's going on around you. Notice the sounds, sights, and smells. Do a quick scan of your body and pay attention to how it feels.

With regular practice, you'll increase your ability to focus--which is tough to do in today's noisy world. You'll also be able to enjoy each moment when you're not distracted by yesterday's problems and tomorrow's worries.

3. Identify three things you're grateful for.

Counting your blessings--as opposed to your burdens--immediately improves psychological health. Studies show gratitude increases happiness and reduces depression.

Make gratitude a daily habit by identifying three things you are thankful for. Your gratitude list could be as simple as feeling thankful for the clean water that comes out of your faucet or appreciating the warm sun on a cool day.

Studies show your brain will physically change when you make gratitude a habit. Over time, being thankful becomes like second nature and you'll experience benefits ranging from improved sleep to better immunity.

Do Your Mental Push Ups

Every day is an opportunity to build more mental muscle. Simple, short exercises performed consistently over time will help you build mental strength.

Additionally, pay attention to the bad habits that rob you of mental strength. Feeling sorry for yourself, giving away your power, and resenting other people's success are just a few of the bad habits that could wreak havoc on your mental workouts. Giving up those unhealthy habits will help you work smarter, not harder.