There are any number of completely valid reasons why a person would be too exhausted to work on a side project after work hours. People do have limitations. Some people have very difficult lives. People do burn out -- and everything does not boil down to having the willpower or not. I'm not saying willpower is irrelevant -- it is relevant. It just isn't the only variable in the equation.
For me, it was a serious medical issue that was partly psychosomatic. Exercise and meditation were just parts of the recovery, and afterwards they stuck around as valuable tools.
For others? I recommend you stare death in the face and use that as motivation. One day you will be dead. Accept that. Then decide whether you want to idle through life letting your mind control you and drive you to do whatever is easiest and most comfortable for it because evolution taught it to seek comforts out, or if you want to accomplish your dreams.
If you want to accomplish your dreams, and know you don't have the willpower, the first obvious step is to build that willpower.
Every day you falter, stare death in the face again and get back to daily meditation and exercise until you've got the willpower to take on your dreams.
Unfortunately, friends often get in the way of that change. Few people do it on purpose, but if you suddenly find willpower your friends may feel bad about themselves and then try to pull you back down. You don't need to dump all your friends, but there is certainly value in evaluating each relationship and making sure it is positive and healthy. Some people you may decide not to see again (hopefully not), and others you simply decide to spend less time with. The saying I like to use is you are the average of the 5 people closest to you.