Helloooooo
Victoria, lots of encouragement on here....but some of the info is not quite right (though well-intentioned).
Firstly, regular frequent pooing is a very reliable sign that all is well. It is simply not correct that a breastfed baby in the first weeks does not 'need' to poo and can go as long as a week or more - this is fine after the first weeks, but in a baby of your baby's age and weight, I would be looking for several soft yellow poos every day as a sign enough is going in the top end.
Secondly, deliberately feeding on one side only is not a way of getting more calories into the baby. With a baby whose weight is causing concern, it helps to deliberately feed on both sides when you can...the more often milk is efficiently removed, the more milk is made. Please don't only feed on one side....try to feed on two sides (or even three!), following your baby's lead, by swapping when she seems to lose interest and stop sucking on one side.
Thirdly, the charts are not based on formula fed babies, but on babies whose feeding method is not differentiated. In any case, charts based on breastfed babies only show a faster weight gain in the early weeks.
I have no idea if your baby needs to take in more milk. Her weight might be fine for her. But if she needs more milk, you have more milk - in your breasts If she needs to gain weight, then feed her more often, especially at night. If she's feeding fewer than say, 8 times in 24 hours, there is scope for several more feeds.
You are absolutely not useless, but I do wonder about the uselessness of whoever has told you to supplement without exploring more about how to fix your breastfeeding.
There is a feeling among some HPs that it's ok to supplement if it's Aptamil. This is not a magic potion. It's a bog standard infant formula that is marketed to make it look different - it is heavily marketed at healthcare professionals and at mums who start off breastfeeding. Whoever has been caring for you has fallen for this marketing bollox, and needs to go on some in-service training urgently before they mess up more women's breastfeeding.
Ask whoever has advised you to supplement what they think of you simply feeding more often....and then reviewing the situation after a week or so of that.
Hope this helps