Cant get it up with this 1 girl its killin me?

here we go let the slaggin begin but fcuk i recently started seeing this girl shes like a model buitifull girl even though shes not as or bold as the girls i usally with in the sack can any1 tell me y i cant get it up with her i usally manage to eventually but fcuk me its embarrisin i know its me and not anytin to do with her but i have been with 3 r 4 girls since i got home from travalling a few weeks back and been with plenty of girls wen was away and never ever had this problom even though my ex is due a baby i tought this mite be it but i was with a girl the othor nite and no problom so any1 shine a light on what my problom is with this girl i do like her its only starting the relationship notting serious im not cheeting or anyting