Need some advice

Right now babe and I are really struggling with breastfeeding, at 2/3 months I was told to start topping up with formula because she wasn't gaining weight adequately, she was still gaining just not as much as the health visitor wanted her to. She's now 5 months, gaining better but about 2 weeks ago she refused breast altogether and has only just starting taking it again the past couple of days for a couple of minutes before wanting a bottle and I just dont know what to do. I don't know what I'm even asking for haha, I'm at a loss of what to do and what the problem could be, this is my last baby and I just wanted so badly to breastfeed since I couldn't with my other two and I'm feeling a little down about it.. Sorry this turned in to a long post/rant, thanks to anyone that takes the time to read it.