It's so hard when you get these feelings. I think that while the whole menopause issue is widely debated, the empty nest is swept under the carpet. All one can do is battle on - I never thought I'd suffer from empty nest but have had had two episodes of it - the first when my daughter moved out and was about two and a half hours away. She then moved home a few years later for about six months. I then had another bout of it after she married and contact was much diminished. We lived about an hour and a half away from her at this point. We were like you, we lived in a beautiful area of the country and had a lovely home and garden. But I was so unhappy. I even considered going back to work (I was retired by this point), had lovely longstanding friends, but nothing helped. My daughter was an only child, (I had her in my thirties) and it was so hard to see the in-laws take over (they lived much nearer) - even to the stage of going on holidays abroad with them. It wasn't ever a money thing - we have been very generous towards them.
Then one day I drove to meet her for coffee - and she said how much she missed us and would we consider moving? I had misread the situation and hadn't realised how she had been feeling about not seeing us often. The upshot was that last year we moved nearer - we're about half an hour away now. It's great - and we now see our grandson who was born around the time we moved.
There's no easy answer for these issues, but as others have said, be kind to yourself. Try and find some nice distractions.