Firstly don't feel guilty. You have breastfed for 3 weeks which is more than a lot of people manage. You may also find you can continue to mix feed, maybe breastfeeding at night/evenings and using formula/expressed milk in the daytime.
With regard to amounts of formula, the amounts on the packet are a guide only, not a recommendation. Individual babies will take different amounts, and it is important to feed formula on demand rather than by sticking to specific amounts. Just give her however much formula she wants and don't try to encourage her to have more than she wants. The guidelines also assume a fully formula fed baby - bfed babies often take less volume to begin with than those that were ff from birth.
When swapping to mix feeding or fully formula feeding, you should drop one feed at a time. So you might replace the first feed of the day with formula. Do this for a week or so to allow you to adjust, and the drop the next feed and do the same. As you've already had mastitis you should probably take this slowly, as you want to avoid any engorgement as this could lead to the mastitis reoccurring.
You might find that if you keep the evening and night feeds as breastfeeding that you can continue with these whilst giving formula during the day. If you stop feeding at night altogether then it is possible that your milk supply may drop quickly.
You might also find that your baby comes to prefer the faster flow of milk from a bottle. This might make her refuse to breastfeed, so if you want to continue with some bfeeding you could try some of the makes of teats that are meant to mimic breastfeeding.
Finally, you could ring one of the breastfeeding helplines for more advice:
La Leche League
National Breastfeeding helpline
Also have a look at the Kellymom website which has lots of v helpful advice about all aspects of breastfeeding.