I love being a mom! What job ever gives you those hugs, smiles and giggles? What job allows you to have the flexibility to handle things the way you want to? Whenever there was something frustrating or negative, I would always look for the positive. Now that my kids are grown there are still negative things, like them not wanting to spend every waking minute talking to their mom ... ha ha. I just look at how happy they are and how grateful I am when they do call.
You have to look at what is so difficult about being a mom and find out why it is bothering you. Start making a list of what you do like about being a mom. What was it like when you saw them take their first steps? Write down some of their cute phrases in a journal. What makes them giggle? What do you enjoy seeing them do? What are you able to do that other people are not able?
Do you surprise your kids with fun things to do every once in a while? If you are not creative (I am not), go online and Google "fun things to do with kids at home" or "fun surprises for kids." Surprise them with a fun dinner. It just sounds like you are not adding fun to your day.
Do you hug your kids a lot - just for the fun of it? Do you "steal" kisses? Bring more of the touching, emotional stuff into your life.
It's just like when our marriages get a little stale. You just have to plan some fun again and more touching.
And whatever you do, do not put a percentage on how much you enjoy being a mom. Just keep focusing on the things you like instead of the things you don't. It's hard to think of two things at once, so keep replacing the negative things with positive things. Go to your list of positive things often in case your brain is so full of negative things right now. Keep lists everywhere you go. Post pictures of your smiling kids everywhere in the house.
Enjoy these years. You can't get them back.