do NOT feel guilty,,,not every mother nor every baby can breastfeed. i tried breastfeeding my twins and made it 3 weeks (i ran out overnight). i knew my babies were NOT getting enough milk and put them on formula. the nurses made me feel like dirt and had me in tears for so much as calling me a bad mother because i was switching to formula. do what is best for YOU and YOUR baby and do NOT look back. it is a personal decision and only you can make it.
as for the sickness thing, "tis the season" with fall many things can cause the sniffles. it may have nothing to do with you breast or bottle feeding. my children had 3 weeks of breastmilk and are pretty much healthy, we have no known allergies, no chronic diseases, they are intelligent normal children, with NO sign of ever being overweight. i have a neice and nephew that were exclusively breastfed for over a year and they both are chronically sick with colds, sinus infections, sore throat, and one has a "death" allergy to peanuts. there is research for the pros/cons of both, but in my experience i think alot of it is "luck of the draw".
do NOT let anyone make you feel guilty and especially yourself. trust me i have been there done that and it was horrible. but if you have a happier baby, you are happier and more relaxed all will be well. a fussy baby and a stressed out mom are a bad combo. do what is best for you and your baby whatever that might be and be confident in your choice, you are the mom,,,and you "just know" ,,,its a "mom thing"