MN really can't help you. Not this board, not the Relationships board, not any board here. Seriously. No one here can offer you the help you need. If you have spent thousands on fertility treatments over the past couple of years, you can surely afford to spend money on private counselling if you don't want to pursue the NHS route.
Your thought processes seem to take you to some very dark places. You think a sibling is "the most important relationship" and yet you yourself are an only child. So is it something in your own childhood you are mourning and resenting? If so, why are you holding onto that?
You think your friends would enjoy how unhappy you are. Honestly, you really believe that? If they are your friends, they would probably feel only compassion for you. The fact that you think they would enjoy your unhappiness is very telling.
You have been told that your secondary infertility is due to a male factor but you don't believe it. Why on earth not? I would think the doctors have a better understanding of the issues involved than you do. It just seems that you want to blame yourself.
As for siblings helping each other when their parents become older, let me tell you it doesn't always work that way. I have siblings who have been worse than useless in coping with our parents' illnesses and the death of our father. In fact, their lack of support has been a source of great pain to me. I often think it would be easier not to have siblings at all than to have people in my life who cannot be counted on when the chips are down.
And one last anecdote. My DH is an only child, and both his parents were only children. I have never known more sociable, outgoing, friendly people.