GTD side effect
I don't know if I can link it with GTD. The among of stuff I process on a average day is amazing since I'm using this system, at work and at home. Even if I feel more in control, at the end of the day, I'm exhausted. My mind is definitely not use to thinking "next action" and "successful outcome" during processing and organizing. Is it a normal side effect of GTD at the beginning?
You use a new system and you get an "amazing" amount of stuff done. It is quite normal that you are exhausted if you essentially change your load profile.
Limit your "doing" time.
Limit your "doing" time. You can schedule some time for relax, walk, contemplation or creative thinking.
Great job! Two thoughts. First, The exhaustion you feel is not uncommon. I've been practicing GTD for a few *years* now, and I still have to make myself take a breath before facing my inboxes. And yes, making decisions is hard work. But it's much better to do it on the front end when things are manageable, rather than when it's emergency time.
Second, you now have a concrete measure of everything you've chosen to let into your life. This can be overwhelming, because you've probably not had a solid sense of it before. Now's the time to take the next step (and the real thing that this kind of self-management practice enables): Think about what's really important to you, and start steering your life in that direction. I strongly suggest you read Koch's "The 80-20 principle." Yes, you can't swing a dead cat without running in to a blog that talks about it, but you really should go to the source. Tim Ferriss's book is a modernization - read that if it works for you. But trust me, you'll definitely look at your work and life very differently afterwards.
Hope that helps!