I definitely do not know (and I'm not asking you to justify at all!) the reason for switching from breastmilk to formula. And please, please forgive me if the following is stuff you already know. Just in case no one else in your family or circle of friends has breastfed past 3 months:
1) 3 months old is a HUGE growth spurt time for your baby, so even if you feel like you might not be making enough milk (if that's the reason you're considering switching to formula), please know that you likely ARE making enough milk and it's just that she's trying to increase your supply to match her needs (normal, happens just like this, is biologically really cool!)
2) it's ok to breastfeed past 3 months (even if other's are giving you a hard time about it)
3) a really good, double electric breast pump runs $240-400 bucks -- this is WAAAAAY cheaper than formula will cost you over the next 9 months, so if you want to continue to give her breastmilk, you can (or even 1/2 breastmilk, 1/2 formula - you'd still be saving $$)
4) breastfeeding DOES get easier with each passing day, and I'd say the first 4 months are the most intense - if you're just feeling too burned out, please know that it's not always like this, the breastfeeding relationship changes as quickly as your baby grows :)
5) if switching to formula is still your desired plan, I wish for both of you a very smooth transition, and keep an eye out for signs of intolerance/allergy (gassiness, eczema in the form of red/rashy cheeks and dry/red patches of skin, sleeplessness, increased fussiness, mucousy or green or foamy stools, small streaks of blood in stools, etc.). Cow's milk and soy, the 2 things formulas in the grocery store are based off of, are 2 of the "big 8" allergens (90% of people w/ food allergies are allergic to these 8 things: cow's milk, soy, eggs, wheat, peanuts, tree nuts, fish, shellfish http://www.foodquality.com/mag/10012006_11012006/fq_10012...
). If your baby would show any physical signs of discomfort due to the change to formula, contact your pediatrician for hypoallergenic formula. This is not said to scare you - my son was not on formula, and he was allergic to cow's milk products via my breastmilk! But I have lots of friends who formula feed, and several of them had babies with the same issues as my little guy, and it turned out that it was allergy too, and hypoallergenic formula was what helped them.
Congratulations on your new little girl, and don't stress about what/how you feed her - what matters most is the quality time and the love that you give her! :)