When only one new habit change is attempted per month, success rates can be as high as 80%.
However, when people are ambitious, and attempt more than one change simultaneously, their success rates drop to below 20% per habit.
By focusing on a single habit change, you have fewer distractions, and more available motivation and willpower to propel you towards success.
Be specific and realistic about the goal you choose, and write it down for best results.
If you need help establishing what is realistic, and need methods to get you there, consult a lifestyle coach or trainer that specializes in your needs.