Writers are told often to kill their darlings.
This doesn't mean we all need to end our novels with planet-enfolding nuclear holocausts. It means that you have the bollocks to cut the scenes you love, but don't need. Slice the dialogue tags that make you smile, but aren't necessary. Exsanguinate the paragraph you feel is the perfect zinger, but all your CP's are saying is dead weight.
It's a hard lesson, killing those darlings.
Here's a harder one- hurt the people you love.
Yeah, that's right. Hurt 'em. These fake people that live in your head who you've nursed to life and massaged dialogue into for months or years need to be in pain every now and then. Let them step on the glass on the kitchen floor. Make them burn their hands on the stove. Allow the water to boil over and scald that first layer of skin off. Slide that knife right through their thumb.
Can you tell I'm not so good in the kitchen?
Beyond physical pain, if you want a truly realistic and moving story arc you are going to have to allow bad things to happen to them. Their husbands cheat. Their moms die. Their kids get sick. Their cat eats their guinea pig (I have a big cat). Their dog gets hit by a car. Their car gets hit by a dog.
You name it. Make it happen to them.
Of course, not all these thing at once. That would be one hell of an odd story. But you get the drift.
Don't step back so far from your perfect shining character that you don't want to make them suffer. Suffering is part of life and part of their journey, if you aren't willing to convincingly hurt them in order to sell the story then you're not the writer for this story.
Don't go easy on them just because you love them.
I started taking kickboxing lessons a couple of years ago, towards the rounding up to 40 part of my 30s. I am not awesome at it. I never will be. But I like kicking and punching things, and my instructor likes to piss me off so we get along well.
When I landed my first hit (a right hook to the kidney, thank you very much), I was so ecstatic that I threw my hands up in the air and yelled HOORAY!
And my instructor punched me right in my unprotected face.
"Never, ever do that again," he told me.
I am a much better defensive boxer for having been punched in the face, and my instructor is good at his job for not having any qualms about punching me. It's a good lesson for writers, too.
Don't pull the punches on your characters because you care for them. If you're crying when you wrote it, chances are a reader or two will tear up as well.
And that's awesome.