my formula fear

My 3 mo old is EBF and I want to quit so badly but I keep going for her. My goal is 6 months breastfed but when I think about her switching to formula I'm scared it will change her? Is this a legitimate fear? I wonder if she will gain a ton of weight, start getting sick, that she will stop progressing as quickly as she has been. Parents who switched to formula- what is it like?