Love = a decision. An action with pure motivation with no fear. No feelings. No co-dependency. No self importance. No ego. Few men and women reach this state. Think Buddha, Jesus here etc.
In love = human condition = us = co-dependency, emotions, an action with motivations and fear. We act or enter in any relationship because of our self interest, may it be friends, family or love. We do it with a motive: it makes us feel good, accepted, or because we feel guilty etc.
We are dysfunctional so we always lack the energy to be in the state of "joy". But instead of looking into the mirror of our relationships with others which lies within: our consciousness. We seek distractions such as status, acceptance, money and being in love which are only a temporary boost of energy. and we named all of those happiness.
In love is a drug. We seek its high. And many people always want to remain high otherwise they hit bottom which is depression.
The answer is you don't love her, you have never loved her. You needed her for your own self, emptiness, loneliness, expectations, status etc. and now that your boost of energy is gone, you no longer need her. You've used her But the reverse is true also.
Love in men is merely an arrangement. As long as it benefits both of you guys, it's good. In your case, it no longer benefits you. Because you guys are not married and no kids are involved, I hope. Don't walk away, run and run like hell.
Go into hiding. Spend some time to look into your own consciousness. It's not love that failed you. You perception has changed. I'll stop for now.