The idealism that your partner may bring to the relationship is drained through their interactions with you, and it may be hard to figure out why this is happening. They might not have even noticed the effects yet -- and they may never -- while you're only dimly aware that something is just off between you. The problem is that you both try too hard to be involved in each other's emotional lives, and you fail to set up boundaries.
Your energies get drained as a result, especially if you tend to argue a lot. You're overly sensitive to this person on an emotional level; you sense it when they're feeling down or hurt, and you take the responsibility for those feelings onto yourself. You might have even started beating yourself up emotionally for causing the problem! This isn't a good idea, obviously. Any emotional disconnect or oversensitivity isn't either of your faults; it's just an unavoidable fact of this particular relationship. But trying to bridge the emotional distance could prove difficult, or even futile. It might be a better idea just to stop trying. If you can accept that there's a level of emotional disconnect and discomfort between you and just move on, you might find that it's not such a problem that you thought it was.