The question "How do you know you aren't making enough milk?' is really important.
When I started nursing, I thought I wasn't, but my kid was doing fine, gaining weight properly -- which is to say, you don't necessarily know.
But if you really want to be nursing long-term, the formula has got to go.
If you supplement with formula, you will decrease your own supply, which sets off a vicious cycle.
I would suggest spending two days going cold-turkey on the formula and all bottle feeding, even your own pumped supplies.
Just nurse the baby on demand, as long as it takes, as often as it takes.
One thing to remember is that baby's stomach is tiny, and breast milk digests quickly.
There were days when basically my kid must have been nursing every twenty minutes or so.
But if the baby needs to nurse again immediately, that doesn't mean you were lacking milk, it just means you need to nurse it again.
(Eventually, these spaces between nursings do stretch out. Honest.)
Another trick for increasing supply is to just leave the baby nursing on the first side until the breast is really, really emptied.
As long as there is suckling going on you will still produce a little thread of milk, so let the baby nurse as long as it is willing to.
Only switch to the other side if the baby actually stops nursing on the first side and fusses for more milk.
When the baby is done nursing, now pump the second side completely dry, even if the baby did nurse on it.
The purpose here is two-fold.
First, to really make it plain to your body that the baby wants more milk.
Any time you don't get absolutely all the milk out, you are telling your body that the milk supply is adequate.
And suckling itself stimulates milk production too, so let that baby hang out on the breast even if you think you're all done.
Second, the last milk in the breast is hind-milk, and it is richer and more satisfying to the baby than the foremilk.
If you switch sides too soon, you are filling up the baby with the more watery foremilk from both sides, but never getting to the more food-like hindmilk.
This will leave the baby hungry and needing to nurse again sooner, even though you have filled its stomach.
Anyway, it only takes about a day to bump up your supply.
If you keep nursing you will notice the occasional day when your kid has a growth spurt and supply lags demand.
Just keep nursing, even though finally you may be producing only a thread of milk, and your supply will kick in to handle increased demand.