When someone tells you "you can't"
Let's talk about reality for one sec. There's nothing real about it. When someone tells you you can't achieve something, they're projecting their reality, what they believe to be true about themselves, onto you. And that's fair when you think about it. Right? How are we supposed to make conclusions without past knowledge? I am constantly projecting onto other people and other people are constantly projecting onto me.
What do you do when someone tells you 'you can't'?
You tell someone else they can. You tell yourself you can. You bring the polar opposite energy into the world, and even further, into the universe. Why? Because that's the law of attraction.
The law of attraction means the universe mirrors your thoughts and actions. Focus your energy on positivity and you will receive like energy. Focus your attention on negativity and that's also what you'll get. By creating positive energy, you are sure to get it back. Do it when you can. And be easy on yourself when you cannot. Do it simply because it's the right thing to do, not only for you but for the good of humanity. Creating and projecting positive energy helps the collective. I know this to be true because I've seen love permeate. Here's a raw article where I share sentiment on love permeating.
How to believe that you can
Picture your desires vividly in your mind. Surround yourself with likeminded people that share a passion and ambition similar to yours, in either intensity or direction. Love, love, love, love. I can't stress that enough. Distance yourself from those that contaminate your energy field. Whether it's a little or completely, that's up to you and what your intuition is saying. Express what you want. If you want a raise, if you want to travel, a partner that values you, don't be shy. Tell people, tell the world. Because if you don't ask you don't get. When someone tells you you can't, think of all the unimaginable crazy-ass things that are happening and have happened in this world and think about why in the hell can't you?