What do you want to do?
Do you want to go out with this girl?
If you do, will you still pine for the other one?
It is possible to be attracted to more than one person at a time, but what you do about it is what counts.
Say you decide that you stand no chance with the older girl.
You could move on, go out with this new girl, and put the first girl out of your mind.
Or, you could continue to pine for the first girl, go out with the new one because she shows an interest in you, but not really be interested in her.
Option 1 could be good for all of you.
Option 2 could be not so good for anyone.
If you make your move, be sure of your hopes & intentions.