Love is rational, logical. Love is math.
Love is a social emotion that performs an important role in a specific reproductive strategy. Given the state of the world, that strategy is the best strategy, and most likely to produce offspring who will benefit from the future, who will live forever as cyborgs or computer backups or brains-in-vats. As such, it is rational to chose love.
And love is a choice. Choose to love someone. Choose to show that love. Hold her image in your head, then turn on the emotion of love. Making a love face will help: soft, caring, giving, forgiving. Wrinkle the eyes, smile, raise the brows slightly, inwardly, wistfully. Do this regularly, so that it is automatic. The human brain rewires to make frequently used pathways easier to access.
Love someone who deserves to be loved. This will make choosing love easier. Love someone rational, who will see the good sense in choosing to love you. Then give her reason to love you, by choosing to love her and choosing to show that love.
Believe that your partnership will be good for her. If you don't believe that you will be good for her, you cannot coherently love her and want to be with her. If you don't believe that you are good for her, choose to love someone else. If she does not believe that you are good for her, you must still choose to love her, and because you love her, you must respect her choice. Choose to prioritize her happiness, be happy for her, and let her go. You don't have to be happy, you can be sad and depressed and feel worthless, but you have to trust the choice of someone you love. And later, you can choose to love someone else. That is rational.
Don't be romantic, that's irrational. This is not a fairy tale, this is math. Small gestures that show love matter more than grand displays. Grand displays are inefficient: they are more about communicating love to third parties, instead of to the object of your love. Grand displays are common because they are easy, but most humans can see through them.
Show love in small things. Love is composed of the small things. If she wants a cup of water from downstairs, go get it for her: she deserves it, it will make her happy, and that should make you happy, because you have chosen to love her. Love the parts of her that infuriate you. If she is messy, be infuriated by her messes, but also love them, because they are her messes. When you encounter a mess, turn on the emotion of love. Make a love face to help.
Choosing to love someone, and showing that love in many small things, will create efficient displays that do not waste signaling on third parties. Giving her reason to choose to love you will ultimately create stronger signals to third parties anyway.
Love can end, don't fear the end. Dwell on it, and turn on bad feelings as you do, and make a sadness face. Then be glad that it hasn't ended, turn on relief and joy and make faces to help. If the end ever feels good, accept the loss, be honest, and move on. The network of the brain associated with one object of love can be corrupted beyond repair, such that it cannot be activated without activating other conflicting networks. The end should feel bad, but feelings don't change bad math.
Sadness faces should not be trite. The emotion to feel at the thought of loss is despondence, forlorn misery tinged with numbness. The face is blank, the eyes unfocused, the whole body slouched to match. The rage in the moment of loss is too exciting, and will mislead; better is the numb after-sad, that is the emptiness that should be associated with the end.
Contrast with love face. The love face is light, the whole body is light, as though made of clouds. Not excited, not energized; calm, centered, peaceful, satisfied. The eyes are slightly lifted, still stare into space but see a more promising space.
Practice turning these feelings on and off. Focus on objects worth choosing to love, and turn on love. Focus on the loss of those objects, and turn on sadness. Rehearsing these is what it means to choose love.
And it is rational to choose love. Love is logical. Love is math.