My 5 year old stepson gets easily frustrated, then turns depressed and violent

Background, my DH and I have been married for two years. I’ve been in my stepsons life since he was 18 months.Describing SS’s mother as a helicopter parent is putting it lightly. For example, if SS falls and scrapes his knee during a tball game (and is obviously fine and his coach tells him to get up and keep playing) she will run out in the middle of the field, pick him up, and pull him out of the game for several minutes. Most tball games are spent with him in her lap.SS is in kindergarten now, and we’re trying to teach him to do things more on his own like tie his shoes.If he tries to do something like tie his shoes one time, and he doesn’t get it, he’ll start crying and screaming saying “I’m just so stupid, I’m the worst!” And screams, and cries, and is inconsolable for several minutes. Like nearly an hour. He will say things like that, throw himself in the floor, throw objects like his game system across the room, etc. all while screaming that he’s a failure.His father and I try to calm him down and say things like practice makes perfect, no one is good at things right away, and try to encourage him and acknowledge his feelings. But he is unconsolable. It’s like he won’t hear you, it’s unlike anything I’ve ever seen.The teacher has also told us recently that he has these meltdowns in class if he can’t get something right the first time. His mother will just say that he gets that from her, or will try to blame us for the situation by saying we put too much pressure on him.He’s always been a little like this, but since he’s started school it’s accelerated so much, to the point where he has at least one meltdown a day. It’s all very traumatic, screaming, throwing things, putting himself down and everything.Our situation is especially difficult because he’s with his Mom every day except a couple hours during the week and every other weekend. She has full custody. She doesn’t see this as a problem, but it’s definitely a problem that I only see getting worse in the future.We’re clueless on how to handle this situation, I want my stepson to be able to handle his frustrations in a more positive way, we’ve done everything we can think of to try to solve this problem, but it’s only getting worse.I’m very worried and at my wits end. Any advice is appreciated.Thank you. via /r/Parenting http://ift.tt/2qzIzNn