"I love him/her so much it hurts"
This is a very poetic and emotional way of expressing deep love and longing. It conveys the intensity of the feelings, almost to the point of physical pain.
Here's a breakdown of the phrase:
* Lo quiero mucho: I love him/her a lot
* Que duele: which hurts
The phrase captures the bittersweet feeling of intense love that can sometimes feel overwhelming.