How to let go of a dog you love?

Well I found a ding and then it ran away so an hour later it came back! I let it in my house, my mom won't let me have dogs inside so it's staying in a room. The dog was freezing cold from running around in the snow so I got him a blanket. My mom called animal control because she wanted to find tue owner. control said that they had to by law puck it up but the girl said she would let us stay with it for a couple days. I stayed with the dog for 8 hours and she loves me. She is always coming and cuddling with me and when I leave she whimpers. I know it will have to leave soon so how to I loose my attachment to it?