Why am I losing boner mid sex? I'm only 2

I am very easy to get hard when she touches me, or even when she's speaking up close. However, after a while, I am unable to get a sex.

I'm just 24 and have a very active lifestyle. I train at least 4-5 times per week I run three to four times a week, eat a balanced diet and consume supplements (vitd3 magnesium, vitd3) I never had to deal with this issue as the last year when I was with my former.

I stopped watching continuous porn about two years ago. I do watch it each 3-4 times a year. I don't ever masturbate. I don't understand why this happening to me.

I've even tried cialis(megalis 10 mg in India) it worked well initially however the second time I lost my erection after the second round.