"Whatever the mind can conceive and believe, it can achieve."
Napoleon Hill, ~1930
The subconscious is always working; if you can believe it, you can make it happen Hill believed the thought isn't about just mere wishful thinking. But the real belief in oneself to be able to accomplish such goals or change, combined with the determination and discipline and sustained effort, is the foundation of success in one's goals.
Try this
Write down one thing you want to believe about yourself. Say it out loud three times. Notice if your body resists or accepts it. It is okay to dream big and have big goals, but understand that big goals come with big sacrifices. Do not limit yourself