Sexual problem during sex. 1-2 months.

I have been sexual relation with 30years old woman from 4years,she's my first sex.I sexed with her so many times when I fuck her she always told me your penis is not erected enough but I sex with her.sometimes I satisfied Her sometime not.when I sex with her I getting harry while I thought I Wil not find it again in this duration I fuck her fast some time she told me to do fast because we meet after 2-3weeks or 1-2 months.during sex after10-15 shots my sperm about to come then I change posture then again after10-15 shots I do same..by this I can sexed with her half an hour to hour then my sperm come out then I feel weaker and tied..she told me her husband never get tied & his penis erected enough he satisfied her...right now I m going to marry a girl now I m thinking if I don't satisfied her what I'll do.it is frustrating moment for me even I'm not concentrated my work.please help me & tell me good in sex or not & remedied me.