For the fellas: While having , how can you keep goin w/o yo gettin soft afta a pre nut?

i have wit my wife we been together a few yrs at first tha was great i got her pregnant and errthang. now seems like when we have ill prenut quick then my will get soft a lil and i gotta stop to get it back up then ill go strong. but at first this neva happened i stayed strong the whole time. what changed? could it be me or is it im not attracted to her stuff no more. i neva had this problem wit any other female. i tried pills and errthang.. any fellas who can relate plz give me advice, no funny stuff.