If you're not capable of loving your partner the way your partner deserves to be loved then you can only reproach yourself when this person leave you.
If you can't be the love of your partner's life, the person your partner has always dreamed of and the person your partner deserves to be loved by, then make the difficult choice and let her go.
It will be hard. It will be painful. It will stick with you for most if - if not all of - your life, but it's the right things to do. There is no greater act of love than to love without any benefits to yourself.