I'm having trouble understanding why I'm waking up

I really don't know. I wouldn't even consider myself a beginner, perhaps a higher tier intermediate LD'er. It seems I'm waking up right in the perfect moment of my LD, right after I set everything up perfectly.

I'm sure it's not excitement even though that it would seems like a resonable explanation. I've overcame that long ago... And it's not the dream ending either, I once tried four attemps in a row:embarrassed:

It seems it happens when my mind has trouble predicting how something would happen... It feels like it wants me to come up with an answer. I take a very brief second to think and the thought blacks out everything.

Should I have less focus on my thoughts and more on the enviroment?

You just gave yourself the solution :) You need to GROUND yourself in your dream environment. Your thoughts can come after you are fixed to where you are. Focus on the elements that make up the state you are in.

Try focusing on your breath. When you take that brief moment to think its possible that you hold your breath a moment which triggers your body to wake up. I could be wrong but its plausible, I experienced it before.