Hi
I was in a similar position ( many moons ago now) . My DC were younger when I realised I had to change the dynamics because I had a miserable child and I was miserable too . My husband worked away 3 weeks out of every 6 so I needed to be able to control the atmosphere in the house - and to make my relationship with my second DS more positive.
like you , elder child was easy going , positive, compliant soul. In comparison , the younger one was ( seemed ) difficult, defiant but with flashes of being absolutely amazing .
I made a ticks and crosses chart.This may not work with you as yours are older but it may help you.
I gave ticks for good behaviour and crosses for unacceptable behaviour . And looked out for every single bit of good behaviour . So , it went a bit like this :-
when you walked from school you stayed on the pavement - tick ✅ ( ticked chart with a smile and a bit flamboyantly) , when big DC told a story you listened well , tick ✅, when you came to sit down you sat beautifully on your chair tick ✅, when you were eating you picked up your knife and fork really well ✅, at the table you kicked your brother, that was unkind and it hurt him ( all said in same tone and no dwelling ) cross on chart ( I just had a line for each day and wrote ticks and crosses on each line for that day) . During the day I would ‘ catch him ‘ being good ‘ you waited patiently for your kindle to recharge , . ✅ etc etc
sometimes I had to really look for a positive - you walked into the kitchen quietly tonight , ✅ . Any negatives were mentioned in same tone as positives or at least without angry condemnation ( I’m not saying do this whilst dealing with bad behaviour but afterwards when reviewing the day, the last hour etc)
So at the table , bed time etc I would say ‘ wow! Look at today ! Today you had 5 crosses but 25 ticks! How fabulous are you ! I have really enjoyed being with you today and am so proud of you .well done!
I made a big thing of how many ticks to crosses there were. ‘ let’s count … hmm there are 1,2,3 , 4 crosses but so many more ticks . Let’s count the ticks -1,2,3,4,5,6…..14,15,16 ticks!’ So many more ticks, 12 more ticks than crosses ! How fab are you?! ‘
In the beginning it was sometimes difficult to find positives so I made myself look for tiny ones . It got easier as time went on.
My DS was assessed for autism at 8 - and was given a statement of ADHD , dyspraxia and dysgraphia .
I later realised he is a highly anxious child who reacts with anger , shouting etc when he feels nervous, anxious and unsettled.
We did a lot of social stories about what to expect , how to behave in different situations.
My DS has just turned 21 and is a wonderful, funny, warm , kind human being . We laugh because he shares many similar traits to me :-)
This may not be any use to you ( and isn’t brilliantly explained ) but hope it gives you an idea that may help you . Good Luck , I know how tough it can be .