I know exactly where you are coming from, I faced the same problem earlier. What I noticed was that starting was the most challenging. Once I started writing then it wouldn't be that difficult. So, instead of thinking about it all the time.
I made a rule. Everytime I'd see myself asking the question - if I should write now or later - by rule I just made the decision to start and not think about it anymore and it worked. Later this rule helped me make it automatic. So, I made a schedule to write for one hour every morning. I applied the same rule and it worked. Then I slowly increased the time to two hours.
I also applied this rule to many things that I wanted to do but could not gather enough motivation and it works everytime.http://www.impactlanguagetraining.com