It's constant

I can think of at least three avenues to the same feeling:

A - Having never known something yet still having the feeling that something important is missing in your life – there is a gap.

B – Having briefly experienced a sense of completion and then letting it go before fully understanding it – the void will always be painful.

C – Having a close relationship with someone or something that is suddenly removed from your sphere – the bottom drops out.

All of these lead to a feeling of constant absence.