You are doing your current girlfriend a disservice by continuing to date her when your heart is not in it.
No one wants to be treated to a 'pity date.'
You need to make a clean break with your current girlfriend and then you need to grow up. That's right, I said "grow up."
You need to grow up because you have built this other girl up in your head until you find yourself holding her image up in your mind's eye as a shining beacon of possibility.
This is crap. The more likely scenario is that this older woman will never think of you as anything but a close friend. And the more you attempt to prove yourself worthy, the more she will withdraw.
Please focus on yourself, for now. Take the time to learn to love yourself. Find out what makes you tick. Who are you? What are your hopes, your dreams, your goals?
Where do you see yourself in two years? In five? Twelve?
If your first impulse is to answer and put one of these women in the picture, that is a sure sign that you are not old enough to be with either one - because it is only by fully knowing yourself (and being at peace with yourself) that you will be able to share your life with someone else.