Absolutely don't feel guilty if you choose to switch to formula. The key things are that your baby eats and you are both happy.
BUT trust yourself that breastfeeding is probably going better than you think (given that you seem to be struggling with confidence) and if you stick at it you will succeed. I speak as someone who had milk supply problems, did mixed feeding for 7 weeks at the hospital's recommendation, but then managed to switch back to exclusive breast feeding which continued for more than a year.
If you would like to breastfeed (again, don't feel that you have to - only if that's what you want) then I'd really recommend a lactation consultant or going to a breastfeeding cafe for support and help. Kelly Mom website is great for breastfeeding information.
And whilst I agree with techgirl that getting outside is good if you're feeling cooped up, it really depends how you feel yourself. Personally, I found the first few months weeks overwhelming, and needed to do less, not more.
I'd suggest spending a day in bed with your baby, cuddling all day. Get lots of skin-to-skin contact (take your baby's clothes off, and lift your own clothes), feeding every few minutes if that's what she fancies. Don't try to do any housework or meet anyone, just chill together in bed. Make sure you have a big jug of water and lots of snacks to hand, and lunch in the fridge (rope your partner into this).
You will get past this, just give yourself time and trust yourself. 