Don't know what 2 do!!!

im 21 and my partner is 35....we have been together nearly 6 months and on the 19th june he is goin 2 court 4 access 2 his 2 kids who are 7 & 12.. andhas said he will b goin 4 custordy after 6 months...thing is and this might sound selfish but i dont want 2 have another womans kids living with us i think it wud ruin our relationship and feel the r too old 4 me 2 "play mummy" as he puts it. he also has a 18 year old girl wich makes me feel arkward as im only 3 years older than her my self. any help or justsomeone who has been in the same situation really.. it wud b greatly apprectiated thankz x x x