It's so difficult when you have to get your child to face something when you can't be by their side.
It doesn't really matter that you know it's for the best, and that they will be fine, and they will be ok, there is still that part of you that longs to protect them from one more bit of hurt. I can't compare in terms of nursery drop of becausd my feral children skip in with nary a cursory glance behind them to wreak their own special brand of chaos, but when I had to leave the eldest when he went in for an operation, the look of betrayal he gave me when I made him take the sedative before he even got to the anaesthetic part was enough to break my heart. I was sobbing as I walked away because he so rarely doesn't want to do something so badly he makes a big deal of it. I don't know that I could handle that every day at the nursery gates, so hats off to you to be honest! That bit about taking a deep breath and then going in... I actually got quite emotional just reading it! You are training him to handle big emotions well, but I can imagine it is really hard. He can name.the emotion too, which is a big deal.
In terms of nursery, is there anything you could do to get them to update you when he has gone in?
Also, is there any way you could alter the transition points? So have a different entrance time where there are fewer people, or have him do a special job with the teacher when he gets in, or have him take in some boxes to the teacher so he is distracted? On bad days in the past, nursery has been the recipient of many a vital old shoebox or bit of tat that was of the highest importance to the nursery teacher and just simply had to be put away on her desk the moment she saw him at the gate. That way there can be no hanging around on the gate, because he has a super special important job to do. Or maybe you could time it so he goes in with a friend?
I dunno, watching them cry in pain or emotion is 100% guaranteed to make me weep in empathy with them because it's just one more thing for their tiny little bodies to cope with. I get it! I don't think it is a bad thing, but I know it can be emotionally exhausting. Make sure you are looking after yourself too!