I don't mean to criticize those who choose to co-sleep and let the child lead in weaning, if they do it because they feel its BETTER for their child. However, many people do this because they don't want to cross the will of their child, and allow their child to guide them instead of parenting. This philosophy will backfire as the child becomes very strong willed and demanding as it grows up. It sounds like you're feeling guilt about being away from her during the day and thus giving in by sleeping with her and feeding her in the night time. I'm making that assumption based on how you worded your question, and how you seem to feel like this is no longer working well for you.
After reading all the responses many ladies feel that co-sleeping is great and something other children miss out on. But you have to find what works for you. These women WANT to co-sleep, nurse as long as the child wants and let the child lead them in their development. Many other mothers want to be the parent and direct and train the child. You need to decide which philosophy you buy into or you'll feel conflicted.
I was frustrated by these two sides when I had nursing issues with my infant. I'd get people pressuring me to get her on a schedule and then others who suggested wearing her all day long and letting her eat whenever she wanted. What they didn't realize was she wanted to eat ALL DAY LONG, and when raising a 2.5 year old as well, I was exhausted and needed some sanity. I had to find a balance between the rigid schedule and listening to her needs. Now at 7 months, she is still nursing every 2 hours (or sooner if I'd let her) while awake, no matter how many solids she eats. But she sleeps at least 2-4 hours twice a day and all night long. So it evens out to 6-7 breast feedings a day. She can't be stretched longer then 2 hours, though occasionally with lots of distraction she does this on her own, but I feel its important to train her to not eat on and off all day long. She is on a loose schedule, meaning we do things in a certain order (eating, napping, etc...) but not by a clock. Bottom line from all of this, is that you've got to make a decision on what you feel is best for you child. You know their temperament best.
As far as a good age to start weaning, you've definitely past the 1 year mark, so you are not endangering her by weaning. But it is a special bonding time the two of you have, and since you work outside the home, she will need to have extra time with you when you're home. You might need to end co-sleeping and get her weaned from night time feedings, but keep up the daytime stuff. And then as you wean her from those, find other special touch things to do with her, so she has that bonding time with you.
Also you spoke of her not sleeping through the night as she got older and was that habit. Yes it is habit. Babies are capable of sleeping through the night by 3 months old, and many are when even younger. Some still struggle until 6-9 months old. But the co-sleeping situation makes it convenient to use your breasts as the only soothing option. Babies can learn to self soothe, but allowing them to snack on and off all night long can make it impossible to learn a different option. Does she take a pacifier during the day while you work? What about thumb sucking? The pacifier helped my first child sleep through the night around 3 months old. My second wouldn't have anything to do with it, and didn't really want her thumb either. But she learned (and I didn't make her cry for hours like people assume) to sleep without anything around 2 months old, I'm talking 8-9 hours at night. And when she's going through a growth spurt she'll wake up in the night for an extra feeding, otherwise she sleeps all night long. Usually she'd go to sleep after nursing, but often she'd awaken when I put her in the crib. I made sure the room was dark and had a fan on for white noise, and if she was awake she'd wimper or cry about 10-15 min tops, and then go back to sleep. Not all children are this easy, but they can all learn.
Best wishes as you figure out how you want to proceed. I am not looking forward to weaning either, and have not committed to a specific time frame of when its going to happen, but it will have to be in the best interest of our family. :)