Hi,
This is a fear of mine, I am always careful when I let Jazz off the lead, and If i see another dog on the lead, I will try to put her on it as well, although, this can prove tricky, if shes goes deaf to recall. I always have treats on hand to entice her, but easier said than done if shes spotted someone or something..
I Know the answer is probaly to not let her off the lead at all, but she loves running and bouncing about, i wouldn't like to keep her on a lead all the time..
So to be extra sure i tend to walk her at a time, when I know my local park is quieter than normal, with not so many distractions... this way recall works much better.
Hope you are both feeling better this morning, and don't feel so guilty.