Anxious and Angry 8 year old

My eight year old son is a type I diabetic.  He has always been extremely emotionally immature with not the best impulse control over his emotions - but we have seen vast improvements over the past year.  We have been working with him to think through his feelings before acting, and to make decisions, rather than just be pulled by his emotions.  For many things, especially with siblings, this has been working.  But one area which we are still struggling, is with outside activities that he wants to do, but which require him to think on his feet, and which may impose a bit of stress.  These are activities which he has asked to do, which we know from years of experimenting and trying things, are things that he enjoys doing, and are appropriate for him physically and intellectually.  The problem that he exeriences is if a given activity presents any stress (somethign is challenging, new, or involves a new application to skills he already has, he gets disproprotionately stressed out and then the next day refuses to go - even tho the over all experience from teh day before was something he enjoyed.  We involve him in the activity selection process.  We always review our rule with him that if you choose something, you need to see it through  - then you need never do it again.  We review times when he has had these responses before, and how we have overcome them and the activities were in fact fun.  We talk about the time we let him walk away from something he really wanted to do because it was causing him stress, and how he regretted it for a long time afterwards.  But still, today, with activities that he LOVES so much that he identifies them as his "favorite things" and "things he is best at" among friends, teachers, etc - he can't get comfortable with his decision to face stress head on.  He becomes angry and refuses to get ready for these activities.  We love him so much and worry a great deal that he still hasnt developed the tools needed to make reasoned decisions and stick with them.  Everyone feels a little sense of foreboding before something that is challenging or new - but how do we help him to trust himself, that his choice to do this was a good one, and that running and hiding from all challenges is never the best path.  He is significantly defiant in these moments - such that those who dont know him may not see the underlying anxiety that is driving it - but I have seen him after the storm abates and he is full of regret for taking himself out of activities.  Any help you might have would be hugely appreciated!