My older dc were curious to try my breastmilk when youngest dc was a baby. I expressed into a cup to let them taste. They all liked it, but only one wanted to go on drinking it. I wouldn't have minded giving her more, but unfortunately I could only allow tastes as my supply was never good, so of course I prioritised the baby.
I, too, struggled to feed dc1 (ended up supplementing bottles with breast). TBH it was a struggle to establish bfing with all my babies, but all after dc1 ended up being almost exclusively breastfed until weaning, and I continued bfing them well into toddlerhood.
There is no need to feel guilty. Breastfeeding may be natural, but that doesn't mean it comes naturally to all of us, or even that it is easy! You did your best for your dc1, and gave her what she needed: food and cherishing.
Re the risk of ff babies being over weight. I believe that is because your body adapts breastmilk in quantity and quality to the current baby's needs, so it is very difficult for the baby to over-feed. But formula is more static in its make-up, and, because you can see how much the baby has taken, it can be easy for the parent to slip into the habit of encouraging the baby to finish the bottle, rather than letting the baby stop when she chooses to.
Give your toddler a taste if you like, give her more if she likes, and don't give a dam if others are judgemental. You're doing fine.