Mothers have started feeding (too) early for generations and we have all survived, so don't feel guilty. If your baby is thriving and continues to take the recommended amount of formula, I don't see a problem with that. Some babies are ready earlier and although the current guideline is waiting until 6 months, the 4 to 5 month recommendation from the American Academy of Pediatrics is still in effect for some babies. Now that we are actually researching infant nutrition, the guidelines may change yet again.
One reason for holding back is to allow the infant's immune system to develop so it can protect against allergic reactions or sensitivities from certain foods, and to give the baby's gut time to mature enough to properly digest certain nutrients. We want your baby to thrive in the first six months, not use all her energy to try to digest a food she is not ready for.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.