You are not a failure!!!! It's hard - they don't come with instructions, they're all different, the books don't usually have videos to really show you how to do stuff... :shock:
It's not too late at all - it's a good time to try getting into a routine - doesn't have to be structured, but babies like to know what to expect next in their lives. :wink: First, you said you're worried about your milk supply - why? Is she gaining weight well and having lots of dirty diapers? Those are pretty good indications that she's getting enough. What you want to do is try to give her a good long feed at every session rather than snacking. How long does she usually feed for? Do you use breast compression once she starts to slow down and are you double-side feeding? That can help. If the weight gain isn't that great, there are some options, like fenugreek, blessed thistle or even medication to help.
But start with trying to get a good feed in. If her weight gain is ok, she can probably go at least 2.5 - 3 hrs between feeds during the day. If she's fussing before that, it's probably overtired or overstimulation. Try to limit her awake time between morning wake up and first nap - definitely less than 2 hrs. You want to have a routine for naps, but try not to nurse to sleep or leave in a swing. There are ways to wean this off gradually too! I found pat/shh good to use to get her sleeping out of my arms - have you read the sleep interview? Also, on the night waking forum the Gentle Removal PLan is great for babies that have been nursed to sleep.
Once you get her eating solids meals through the day, it might be easier to start noticing a pattern for her day - have you already? If so - maybe post what her day tends to look like - might help us help you.
Gotta run for now - let me know more how I can help - we'll get this baby sleeping better!!