If you find yourself having to end sessions early, which will gradually lessen your milk supply, you might as well begin using a breast pump. Electric ones are usually more powerful and less time-consuming (it depends on how often your child needs to be at the breast), but they are more expensive. Some hospitals or services rent the electric ones out. Breast-milk is always best, but there are many brands of formula available which do a decent job of providing good nutrients for your child.
When I stopped breast-feeding, it was earlier than eight months, and it was because I had gotten sick and was miserable doing it. I used a hand-pump for a while, but my supply gradually lessened until it wasn't worth it anymore. Many brands of formula actually provide a different amount of vitamins and minerals for children 9-24 months, which you can find next to their infant formulas.
Our pediatrician told us to wean our son from the bottle at age one, however. This isn't to say that your child shouldn't be getting formula or breast-milk once or twice daily though, because it's an important part of their diet and development. But around age one, usually it's cut back while your child is introduced to other foods. During this time, we taught our son to drink from a cup.
I would say switch to a pump for a few months, then perhaps get one of those 9-24 month formulas and give it to her a couple times a day as a part of a routine diet of mixed baby foods and cereals. Of course you should ask your pediatrician first. But my advice is to use the pump while cutting back a little in order to let her stomach save room for some new foods, and once in a while try a bottle of water with ten percent juice in it. I believe the age for starting children on cow's milk is one, but ask your pediatrician.
