I have the same issue as well. However, after deep reflection I realized that I am not proactive in certain situations and under particular circumstances.
For example: I worked with a boss who was a micromanager. When I was first hired I was enthusiastic and ready to grab the bull by the horns. Yet, a few months into the job I simply sat back and did nothing.
On the other hand, when I find an intrinsic motivation (Daniel Vassallo talks about this), I become automatically proactive.
What I'm trying to say is, do some reflection. Are you not proactive because you're simply not motivated enough due to externa or internal reasons? Or are you not proactive because of other factors? This might give you insight into how to tackle the issue.
On the other hand, ask yourself this question: why should you be proactive? Is it an end in itself, or instrumental? If there's no goal that motivates to become proactive then is it really important to be one?
Hope this helps!