The first day of anything holds an immense power to feel quite significant. The first day of a special holiday for example, the first day of a marriage and the first day following the death of somebody very close to you.
The first day after you and your spouse split up is likely to resonate with all three of those and more besides.
Whether you left them or they left you, the echoes of what should have been happier times fill the void inside you and for a while, it is normal to feel positively numbed by the experience.
Despite well meaning advice from friends and family, listen to your heart when deciding what to do next. If you need to sit still for a while to digest what’s gone on, do so. It’s not wallowing in the pity of the situation, it’s more like catching your breath and preparing for what lies ahead.