Hi,

I am new to the forum and have some questions regarding concentration meditation. 
I discovered meditation 4 years ago, read MCTB and other books but consider myself as beginner with my meditation practice consisting of about 50h of meditation on my own in this timespan.

After a break I started meditating again since last week, sitting in the train with binaural beats on (mostly to cut the ambiant noise and get less distracted). I am focusing on the feeling of my breath at the tip of my nostrils. 
Now here is what happens: I count down from 10 to 1, 1 step at each breathing cycle. I usually manage to get at best 1 series complete, but find myself distracted frequently. When I notice I am lost in thought, I go back to the breath and start counting again.

This phase lasts for 10/15 minutes, to which point a different feeling arises: there is a feel of clarity, lucidity. My vision goes from black / inexistant to something brighter (grey?) and I can do 3 series of counting without any distractions, to which point I stop counting and focus solely on my breathing. 

Problem: at this point I kind of get excited and I can't help it. I feel like I made a breakthrough and don't know what to do next. So I try to stick to my breath for a moment, to test the stability of my concentration, but then I figure I might have attained the elusive "access concentration" state, to which I hesitate between tryining 1st Jahna or try mindfulness meditation, but then, I don't know what to do, I move around my concentration to feelings in my body, vision, sounds, noting whatever I feel. But all in all I feel like I am just aimlessly wandering around the place and so naturally, I promptly stop my meditation session.
So can anyone guide me as to where I am and what should I do?

Many thanks! 

I would suggest a bit more patience and perseverance ;)
What you describe all sounds good. Once you come to the point where you get excited and the excitement distracts you from the breath, make a mental note "excitement". you can do that with an accepting curiousity in the sense of "ah, interesting, there is excitement now". what does the excitement feel like? is it in the mind? or in the body? just be with these sensations for a few moments or longer until they lose their power to distract and gently return to the breath. stay with the breath until the the next big distraction comes up, be with the distraction, then gently return to the breath etc.
don´t worry if you have reached access concentration or not. If you read MCTB you know that things might get difficult along the way. so I would strongly recommend to work at least another couple of weeks at calming the mind as a basis for insight.