Freedom, welcome, accepting that you can't make everything 'better' is one of the hardest things to accept.
Welcome Kindsoul. The first point of contact would be your GP. Depending on the severity of the MH conditions they may refer to CAMHS or to a local organisation that offers counselling or signpost you to speak to the school. If you or the GP suspect another condition e.g. ASD you may be referred for an assessment - depending on your area if may be CAMHS that assess or a neurodevelopmental community paed. It is also possible to go private.
You can ask school for help, schools often have staff who can deliver interventions that can help. If needs are more severe you can apply for a plan that can secure more support - the process depends on whether you are in England, Wales, Scotland or NI.
Maitre I would question why a primary aged child had so much work over the summer that they are overwhelmed. Can it not be scaled back? Academics are of little use if you can't function in everyday life. If DD can not cope now secondary is going to be very hard for her. SEN support is where schools offer additional interventions - like what DD has already received. Schools can offer a range of interventions and adjustments - nurture groups, ELSA, counselling, drawing and talking, Lego therapy, social skills group, time out card...
Roxie I hope DD continues to manage school.
Always DS using the anxiety as an excuse sometimes is one of the reason why it's important to push DC and learn the signs as to when you need to stop pushing because continuing would be harmful. DC with MH conditions sometimes can't rationalise something is in heir best interests and need pushing. I know Roxie was advised differently, but common advice is to limit screen time, it often doesn't help to have huge amounts of time on it.