Switching to FF at 4 months

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I have been EBF my almost 4-month-old daughter since birth. I am starting to switch to formula because we have had a terrible time with me having milk oversupply resulting in frequent clogged ducts and multiple cases of mastitis. It just is not working out for us and I am constantly anxious and worried about how long my breasts have gone without nursing/pumping. My daughter is also very large- in the 98th percentile for weight so I am extremely nervous that switching to formula will increase weight gain. Anybody have any experience with this? Did your babies gain more with formula? I am already concerned that somehow I am setting her up for a lifetime of obesity because of my milk oversupply and the amount of milk she is taking in while breastfeeding. 
Thanks!