First of all, everyone has their own opinion, and what works for one person doesn't always work for others. You are the expert on your child, so do what works best for you and your family.
I have an almost 2 year old and nursed him for 20 months. We coslept for the first 3 months. Cosleeping has it's pros and cons, but it's what we did and it worked for a while. Starting with naptime, I started transitioning him to the crib because I wasn't getting enough rested sleep at night with a newborn in our bed. Once he was successfully sleeping in his crib for naps, I started trying to at least start out the night with him in the crib. I do remember having to bring him to our bed in the middle of the night for a while, but slowly we were able to successfully transition him to the crib. I couldn't do the cry-it-out method either.
My supply also dwindled when my period started, when my son was about 9 months old. I was visiting my mom in Mississippi over the holidays back then and she took me to a breastfeeding clinic (headed by an M. D.). The doctor recommended that I start taking 2000mg of calcium and 1000mg of magnesium per day. I had also heard about the herbal supplement called Fenugreek through the La Leche League that also promotes lactation. I think I took 3 calcium pills a day (1800mg), 2 magnesium (1000mg), and 2 Fenugreek (can't recall how many milligrams each had), and I noticed a difference within a couple of days. Also, be sure you're drinking at least 2 liters of water a day for your supply. If you have a pump or can rent one, that will help as well. Breastmilk is broken down and digested more quickly than formula, so breastfeed babies will have a need, not just a want, to nurse more frequently than a formula-fed baby. If you decide to supplement with formula for a feeding or two, it will impact your supply even more. Just be sure to pump an equal number of times to keep your supply up if you want to keep nursing.
As others have posted, I would suggest making sure your baby is getting a full meal of solids before bedtime. Pediatricians no longer recommend adding cereal to a baby's bottle (I think there are some serious issues with it, but I don't recall what they are exactly), so just mix some with breastmilk/formula/water in a bowl and feed your baby with a spoon.
One other thing I remember having to do back then was to turn the baby monitor down at night so I wouldn't hear every little peep he made during the night, which would wake me everytime. We live in a townhouse, his room isn't that far from ours, and he's got a good set of lungs on him so if he *really* needs me, he can sure wake the dead. ;-) I believe I also got up for every other waking around that time, though he might have been a little older when I started doing that.
If you have any concerns about your baby's health or nightime feedings, by all means contact your pediatrician. Good luck and email if you have any other questions!
(I'm a 37 year old stay-at-home mom to an almost 2 year old son. He had reflux, serious sensitivities to things my diet that I had to cut out, and also had an anaphylactic reaction to cow's milk when I tried to wean my son at 12 months! Thankfully, he outgrew that severe allergy at 18 months.)