Thank you so much for your detailed answer! I never realized the yellow bar behind the overtake ICON !
What really confused me it was that after few laps I was able to use overtake for the whole lap and for many laps with the D bar not going down so fast. But I incredibly didn't notice the yellow bar, infact I could feel that I didn't have so much power in addition as it should be when usind the ERS boost. The H Bar did increase slowly , and this is normal expecially when braking , and everytime I crossed the line the D Bar was full again.
I'll look carefully into it tonight, thank you again for your kind answer!
EDIT : Now I understood! The D bar is the maximum ERS you can deploy for one single LAP, for this reason it was decreasing fast in the first lap and then slowly, because infact I was deploying a little % of ERS because my battery was empty!
Thank you again!