9 year old is always so negative

Im just venting here.

My 9 year old is constantly complaining, blaming, angry about SOMETHING. He's just never happy and to him the universe is just against him. The smallest inconvenience sets him off. It's exhausting listening to it day in and day out. He's been in therapy for almost a year and we're seeing someone for potential diagnosis/medication management soon as it's suspected he's got ADHD as well as anxiety. He's also had a tough time dealing with a poor relationship with his dad lately. It just wears on me every day. Yesterday it was because he dripped milk on his chair when he lifted his straw out of his cup. This morning it was because he dropped his toothbrush. Then someone was backing out of their driveway ( they were already doing so before we even got anywhere near that part of the sidewalk) when he was walking to his bus stop so we had to slow down for 5 seconds as the front of their car exited their driveway's apron. EVERYTHING. We have been over many times in many different terms to "not sweat the small stuff" and how getting angry about things out of our control doesnt make anything better nor does it make us feel better, it makes us feel worse. He's got plenty of strategies to use to calm himself down, he just gets so wrapped up in his emotion in the moment that he cant remember to use them. His feelings are always validated but the reaction to it is what he needs to sort out how to just take a breath and move on. I just cant deal with the negativity and constant complaints anymore.