Couldn't maintain erection for long.

Hello practo, This problem has arisen from last 6-7 months. Before that, it never happened, I had some sexual encounters in the past and they were all satisfactory. But now I couldn't maintain myself for long, a little diversion of thoughts leads to lose it all. This is happening during sexual acts when with some partner and even while masturbating,I don't feel that hardness and stiffness and I masturbate in loose erections, from past 7-8 months I have been masturbating daily, before that it was 2-3times a week but now it's daily, and I am doing it while sexting with my g.f., those thoughts and imagination make me go insanely aroused and I then want to masturbate, even if there is no proper sexting, but any talk where any sexual thing comes it arouses me, I have been thinking too much about these sexual things and fantasies. I think I am overmasturbating and it has lead to the loose erections. Please advice and suggest something which is best for me and improve this condition of mine.