This is such a common question and the answer is always the same. Compromise. No matter what your situation (wife, kids, intern, job, etc) you need to compromise time on some things to make time for others. And depending on your situation it will involve compromise from other people in your life as well like friends,social life,wife,kids.
While most commenters here are right about schedules, That can be hard for some people. What I find is most important is communication with your stakeholders (i.e. wife/gf/kids) and maybe not setting an entire schedule for your whole day , but maybe a set time block a few days a week for "pet project work". What is really important here is do the same for your other responsibilities. Like Tuesday nights is "watch TV with the wife and kids" night and wednesday night is "I work on my project" night.
While most commenters here are right about schedules, That can be hard for some people. What I find is most important is communication with your stakeholders (i.e. wife/gf/kids) and maybe not setting an entire schedule for your whole day , but maybe a set time block a few days a week for "pet project work". What is really important here is do the same for your other responsibilities. Like Tuesday nights is "watch TV with the wife and kids" night and wednesday night is "I work on my project" night.
Most important to remember is Keep Trying!