I lost my pup last week to his enlarged heart which was collapsing his trachea. It progressed so fast. I noticed him coughing at rest and panting on usual walks, so I took him in on a Thursday, got dx'd with enlarged heart/partially collapsed trachea with a fair-ish prognosis and was put on bronchodilators and antibax for irritation/infection, no fluid in lungs whatsoever. Exactly one week later, he went into respiratory crisis, his lungs were suddenly filling with fluid, his heart failed, and there was no longer anything we could do. My baby was also all-white. A perfect angel.
I'm so sorry for your loss, I know exactly how you're feeling. You wish there was something you could do, and I'm certain you also did everything you could, but when their time comes, the important thing is that you were there and they felt loved, and they didn't have to go through further unnecessary pain.