Now come along - pull yourself together! Sorry to be blunt, but you have got this all wrong. You cannot go on wallowing in this. Have you any idea how hard it will be for your chidlren if you express (or even hint at) this grief to them?
I always make sure that my AC know what I am up to - that they see me living a fulfilling life so they can be free. And so that I can have a healthy relationship with them into the future. I am in constant whatsapp contact with them - taking delight in their achievements, providing a shoulder to lean on when needed and telling them what I have been up to.
Life moves on and if you do not accept that and deal with it you will be utterly miserable long term.
Do you think you are suffering from a clinical depression? - have you any symptoms of this? Have you seen a doc about this in the past? If this is the case, then you need to seek help.
If it is purely that you cannot deal with an empty nest, then you need to grab life by the balls and start to make plans for this new phase of your life. Life goes in stages and each has its sorrows and its opportunities.
You are lonely because you are choosing to limit your life to the sadness you feel over your children leaving home. There is a lot more out there.
We all feel daunted by new stages in life, but in the main we don't sit around crying. We look at what is before us and make plans for the future. Change is hard, but we need to look it in the eye and find ways to capitalise on the opportunities presented.
I honestly think you should see a doctor so that your mental health can be assessed and proper help given as appropriate.
Here's a recommendation - look up your local U3A - they have lots of brilliant things for people heading towards this later phase of life.
I am sorry if I sound unsympathetic - I am not. I just think that if you let this get on top of you things will be hard for your children - you cannot make them responsible for your happiness - that is a massive burden for them to carry. Look at this new space of life ahead of you and start to write down what you would like to do with it. Do some research about the opportunities out there. You will feel better if you do something concrete. You can do this!