Believing in yourself is the most important step in achieving what you want, if you've already decided to believe you can't then you won't even bother setting a goal to achieve what you want let alone make any steps to achieve it. The ego would rather be right than achieve inner dreams so will even go to lengths to subconsciously sabotage you.
You can even experiment with "I can" vs "I can't" by swinging back and forth while doing something, I did this with my children, we all stood on one foot and said, "I can't balance" and we fell back to 2 feet over and over, when we switched to "I can balance" we were able to hold the balance for a long time. I tried to do this with my partner but it didn't work because he was so confident in his ability to balance that he was unable to really convince himself even for a second that he couldn't. This is the power of belief.
Another experiment I did was kind of by accident. A few years ago while exercising I got to the point where I felt I could no longer keep jogging, I heard myself think "I can't, I can't" and I stopped with exhaustion as I began jogging again I decided to use affirmations, "I can, I can" "I'm powerful" "I can keep going" I went on for a lot longer than I'd previously believed I could, the affirmations gave me more energy to keep going, it amazed me how energized I felt inside! It also made me start enjoying exercise.
From these little experiments and understanding that it is my belief which determines my success I have been able to achieve many things in my life that I first thought weren't possible. I assumed that after 2 kids I would never get back down to the size I was before kids but with a shift in my belief, I did. I thought I was an easy target and that everyone ridiculed and put others down but with a change in my belief I allowed myself to drift apart from people like that and attracted those who love and support me.
I achieved what I thought was impossible over and over but I didn't even bother trying to until I shifted my belief from impossible to possible. From "I can't" to "I can and I will".
At times you'll be working towards goals and they just won't go to plan. This isn't proof that your goal is impossible, it just means there's a better way to go about it that you aren't aware of yet. When your plan fails, don't take it personally, just use it as feedback and ask yourself, if that way of going about it doesn't work, what else can I try? Perhaps you have set your goal too high and as much as you try to use affirmations to achieve your goal, your inner self just doesn't believe it yet, set a lower goal and achieve that first, then you can keep raising your goal. When you start small you get to celebrate every step of achievement along the way.
Sometimes, you may even find that you're thankful for blockages that come your way because you realise that particular goal or that way of going about it isn't for you after all or everything seems like it's falling apart only for you to realise later that it was all really coming together in ways better than you ever imagined.
This isn't just true for goals, this is true for everything in your life. Experiment with this so you can experience it for yourself, it's very powerful. Question your assumptions about yourself and life, become aware of your inner beliefs and change the ones that aren't serving you. When you switch around your thoughts and feelings you'll be amazed with how much changes, you'll attract people, things and circumstances to assist and guide you and everything will fall into place in better ways than you can imagine.
Start with your beliefs, believe in yourself and write affirmations because as Henry Ford said, if you think you can or you can't, you are right.