Is it normal for a 25yr old to have low libido and hard to hav an erect penis even just for masturbation and much difficult during sexual intercourse?
Not normal. Suggest you see a urologist and get your testosterone levels checked.
I'd be crying. Hell no it's not normal! Will you please seek advice. Your family doctor will be the place to start. Please don't delay.
Is in normal for men to get cuts/tears on his frenulum (banjo string) on his penis from sexual intercourse when not being careful how hard you hav sex?
Not uncommon. Tight frenulum can be painful and make sex less then pleasant. Can easily be taken care of by a minor surgical "frenulotomy" by a urologist unless you have torn it down to the base in which case you have performed the minor operation yourself. Good luck.