Is there actually "Plenty of Fish in the sea"?

I've always heard the saying "Plenty of fish in the sea" for relationships... Then why is it so hard to find someone? The last girl I fell in love with and actually felt feelings for is already in a relationship...

Like generally am I doing something wrong? Dating Apps don't work for many reasons, I've finished education so thats also out of the question... What else can I actually do?
Is there actually "Plenty of Fish in the sea"?
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