Why do you get stressed? I mean, what is going through your mind at the time?
Of course the thing is to not stress about it, but how stupid is it to say to someone "Well, just don^t get stressed!" but it truly is that....changing your attitude.
If you could identify those thoughts, or triggers, or feelings, that come at the beginning - while you are still calm - and think through instead of letting yourself be swept away with your feelings...
And what can you do to manage your time better? Pack something in advance to give ds while you are going to beevers, instead of looking for something for him after the swimming will save time. You don't HAVE to go to the restaurant.
Telling yourself that it is ok to be up to 5 minutes late for Beevers - they are still getting settled!!
Consider moving swimming lesson times so they don't conflict - you may have to go on a waiting list, or see if someone else wants to swop.
Get a little battery operated fan for when you are in the spectator area.
When you feel yourself getting wound up, stop. Take some deep breaths in through your nose to the count of 6, out through the mouth for the same. Focus totally on the act of breathing and repeat (in your head) a word - calm, relax, breathe, peace..whatever works for you!
Also - deal with things individually, rather than seeing everything, so when you are feeding dd, that is all you are doing, don't link it to the other things that are next, iyswim.