The sentence, "I can't", needs to be taken out of the English language completely. There is no better setup for failure, then by using these words. When you have these words circulating throughout your thoughts, you have lost already.
I am a huge believer in the power of your mindset. Your mind can take you to your darkest depth or your highest elevation. Preparing your mind for the battle you are going through, or about to go through, is your key to success. Never let your mind go down the rabbit hole of negativity. Block that nonsense out. It will only lead to you failing again.
Instead of using the phrase, "I can't", try saying, "not yet". "Not yet" leaves the door open for work to begin. Nothing in life will be accomplished without work.
-You want that job? Go to school and get the degree for it. Then, climb your way up the ladder.
-You want to lose weight? Work at changing your terrible eating habits that got you there in the first place.
-You want to hit a PR on a lift? Don't skip workouts, eat right, and work on perfect technique towards that lift.
All these take time and consistency. They won't happen fast.
There are going to be times, many in fact, that you will fail. That is perfectly acceptable. These moments of failure need to plant a seed in your mind that will take root and grow into a perpetual thought of how to overcome these situation's. I hope these moments of failure make you angry. I say that because if you have emotion on your side, that will lead you to overcoming failure. Emotion is a great and powerful tool to use to fuel you. But, you must also be able to control your emotion.
So, don't keep telling yourself, "I can't". Say, "not yet", then start asking yourself what you need to do to improve. If you don't know what you need to do, ask someone who has already succeeded in that area. Success only comes from learning through failure and there are plenty of people who failed before you.
Change your mindset and throw that garbage phrase out the window.