u sound like someone i know..
logically,
it isnt fair. but love isnt logical.
so, if u choose to date this new girl, possibilities are,
a) win-win situation:
u may really fall in love with her and move on from ur ex.
b) lose-lose situation:
u treat her as a substitute and she finds out, gets hurt and breaks
up with u.
c) lose-win situation
u treat her as a substitute but later fall in love with her and
move on from ur ex.
whatever it is, u must not let this new girl know that u still
love ur ex!
-QQ