I’m struggling

I can be in the middle of laughing or having a good time and he pops up in my mind and takes my smile away. I was having dinner with my friends last night and had to blink away tears.

Why can’t I stop thinking about someone who doesn’t think of me at all?

How do I stop, I genuinely neeed to know. I’m so tired of feeling sad when he gets to feel nothing and lives as if I never existed.