Can you be guaranteed success in achieving any of your goals? I think so. Here's the key: if you want to be a totally different person by the end of the year, be a smaller version of that different person on a daily basis. The incremental changes will add up.
For example: let's suppose you want to be a less angry person – more tolerant, more patient. Every day, practice being more tolerant and more patient. Make it a habit to count to 10; bite your lip; smile and breathe; etc. Before long, you will be in the habit of getting less angry.
The same goes for any change you want to make in yourself. Start small. Be consistent. Make it a daily practice to be the change you want to see in yourself. Get it right, even if it's a tiny improvement. Don't get hung up on wanting to see the results.
Stay with the practice of being who you should be, and before long – let's say, a year or so - you will have become that person.
Change works “from the inside out.”