Quick post for those struggling with formula top ups. If you are keen to wean off, have a google of medela's supplemental nursing system. I got in to a situation with my DD (now 3.5) where top ups had led to total breast refusal at 10 weeks. Using the SNS over 5 weeks I gradually lessened the amount of formula &, as it allows you to be feeding from the breast while topping up, milk supply gradually built up enough so that she didn't miss the top ups. I went to a v supportive breastfeeding group who helped me with a regime for this & referred me to a lactation consultant who I talked to on the phone.
Might be worth trying to find out where your best local support group is or, if you can afford it, speaking to an IBCLC registered lactation consultant (google the IBCLC website-you can always call one to talk for free). My DD did have a sneaky tongue tue that everyone failed to spit apart from the lactation consultant... They are expensive but in my doula work I have never referred a client to one who hasn't had a positive experience & had breastfeeding turned around for them & saved themselves a tonne on formula in the long run.
The SNS us a faff to use-and takes a lot of time & energy-but it meant we were back to EBF which was important to me & in fact DD still breastfeeds once or twice a day now 
I know for some switching to FF or mixed feeding will feel right & you need to go with your gut in this, but if you are keen to keep breastfeeding many women find an IBCLC invaluable & the SNS thing I mentioned is a genius invention.
All ok here. A now 3.5 weeks. Wondering whether he has reflux, though I know to tongue ties cab cause similar symptoms & he does have one of those. He's stacking weight on (12 ozs last week) as I have the milk supply of champions this time (thank you DD) so haven't rushed to diagnose a problem but he gets v distressed for periods every day, very, very sicky, lots of wind, not keen on lying flat etc etc
How are everyone's older children getting on? DD totally loves A & there 's been no jealousy but she is waking at night screaming for me & much more clingy & sensitive.