What do you do to motivate yourself when you are about to give up?

For me, it works all the time:

If I am tired, I have a look at this:

If I am just sad, I look at this:

And finally, if I need an extra boost, I look at this one:

Instant relief.

Now look again at the first one...

Why does it work for me?

The first image evokes a sense of duty. A duty not only to others but to myself, to keep moving forward, keep pushing a little more, fiercely, to never quit or settle and to avoid wasting my time and energy with unnecessary complains or excuses, from myself or from others.

The second image resonates deeply in my mind because it helped me overcome a depression episode. I made a promise to myself that whenever I felt like getting unecessarily sad, unmotivated or with a weak self esteem I would jump right out of this negative cycle, cold turkey, bluntly, no matter what. I even dress a little sharper sometimes, or do some power posing in the toilet, or reward myself with something nice, when possible.

The third one is paramount to me because it reminds me that no one has the power to make me feel like I don't deserve what I honestly think I do. It's not a matter of stubbornness or trying to prove some personal point. Nothing like that, but instead it's a commitment to myself that I will keep pursuing the achievement of my goals despite any external negativity towards myself. It's something like: "They said he is going to quit, he doesn't have what it takes, he is not strong enough.... But boy they were wrong!"

How do I deal with constant failure in my life?

Failure is a good thing. Unlike success, which doesn’t carry any information, failure is a great source of knowledge. Learn from it, go ahead. If the next result also is one, learn from that, and move ahead. Even if one does not meet success, ones wealth of knowledge will be a good enough compensation, for, opportunity shall arise, sooner or later.

Failures help you to figure out what went wrong in the previous attempts with that you can do better in the subsequent attempts. One more thing whatever you do, do it with passion by this you love the work you do and it won't give you the feeling of failure.

On one hand, failure is relevant.  Christopher Columbus was a total failure in meeting his goal of finding a shortcut to India, and Native Americans are still stuck with the misnomer of being called Indians. However, his role in world history remains significant. 
 
For things that are genuine failures, you can consider them as being excellent learning tools.  I like this quote to illustrate this point:  "Ever tried. Ever failed. No matter. Try Again. Fail again. Fail better." - Samuel Beckett

Failure is a launch pad for starting life, not ending it.  You have unique talents and insights.  Don't undervalue them.  Good luck.

How do you convince your mind that it's okay to fail?

Fuck it.
Just fuck it.

I asked for a girl’s number at a club once. She just shook her head at me. Whatever. Fuck it. I walked away. I’ll get to know other girls.
My first ever girlfriend didn’t allow me to have sex with her because of God and shit. She manipulated me to break up with her so she could be with her ex, and they were sleeping together. Whatever. Fuck it. Bitches be bitches. I’ve had better relationships after her.
I nearly flunked out of college after my first semester. The dean made me go see him and shit. Whatever. Fuck it. Studied harder after that and doubled my GPA. Graduated eventually. Didn’t give a fuck about other kids who kept comparing grades and based their self-esteem on it.
Dad died when I was only 20. Whatever. Fuck it. Moved on eventually.
I struggle with my business. Whatever. Fuck it. I am learning and I am proud of my art.
I make less money than my peers. Whatever. Fuck it. I don’t spend on needless shit like they do.
Wrote some articles. Nobody read or shared it. Whatever. Fuck it. Continued to write.
The girl I thought I’d marry disappeared on me at the end. Whatever. Fuck her. I don’t need to deal with toxic people who don’t want to act like adults.
Not even sure if anyone would give a shit about this answer. Whatever. Fuck it.
It’s not about trying to convince your mind or whatever.
Things go wrong all the time. Nobody is perfect. We’re all just playing along and trying to figure it out along the way. This is a fact. Whoever told you otherwise is nothing but a god damn liar.
Just live. Do your shit. Do your work. You’re not an idiot. You learn shit. Just keep on keeping on and stop overthinking the shit out of everything. That’s pointless.
If at this point you feel like you’ve a ton of questions and you’re like, “But, but, but…”
Whatever, just, whatever, okay?
Fuck it.
It will be okay.