It's easy to find scads of information on heart failure (I really, really, wish they'd abandon that frightful term and use 'heart insufficiency' instead. The heart does NOT fail!)

Basically, due mostly to age, but also to cardiomyopathy or too much exercise, especially high intensity exercise, your heart can develop collagen deposition in its 'substrate'. The walls of the heart thicken as a result, and the collagen makes them stiffer. It's just harder for your myocytes to make your walls change shape as they did when you were in your 20's. So, in your case, diastolic, your muscles may not contact well, but neither do they release fully as they should...meaning the relaxing stroke doesn't let the wall of the atrium and ventricle move away from the septum, and you get less volume with each stroke as a result.

Just as with most heart arrhythmias, people live literally for decades with heart 'failure'. It does depend on early treatment and control over time, so please do google it, read tons, ask questions, and follow your cardiologist's advice (be prepared to challenge him if your new learning doesn't match what you're being advised to do. This keeps both of you honest).