Trouble moving on to a different task

Before You Begin
Pause and breathe. Even if this shouldn't be happening... accept that it is and embrace it with composure and calmness.
Calmness is the only way forward because without calm, there is no possibility for receptiveness and connection. Your child won't give to you any more than they see from you.
Never forget that a strong connection is fundamental, as there will be no correction in them if there is no connection with you.
The Core Problem
The perception of feeling pressured to fail.
What Will Make It Worse
Reacting with frustration, impatience, or imposing demands to switch tasks will grow their anxiety level, triggering a greater desire for control in them and increase their resistance.
What The Child Needs
1. To feel empowered: Involve the child in setting appropriate progress goals and timelines for completing tasks. Empower them to take ownership of their schedule and manage their time effectively.
2. To be able to see the bigger picture: Help the child understand that the next thing they need to act on also needs to start/end on time, and if they prolong their current activity it will disrupt their day. A good skill to build at this time is having them "pause" whatever they are doing, and decide when would be the best time to return to it later.
3. Positive reinforcement: Offer positive reinforcement and recognition when the child demonstrates flexibility in transitioning between tasks. Praise their efforts to manage their time effectively.
4. Predictability and routines: Provide a consistent routine to help the child anticipate transitions between activities and reduce anxiety about moving on to a new task.
How To Have The Conversation With Your Child
Find a time to sit with them in a comfortable place that is free of distractions.
Begin the conversation with empathy, expressing understanding that it can be stressful and frustrating be asked to stop what you are in the middle of in order to move on to something else. Whether it is a homework assignment, a video game, or a craft project - you can see that they just want to do it their best to completion. Then share that it is something that everyone, even you the parent, deals with because it happens a lot in life.
Let them know that since it is never going to stop happening, you are going to help them figure out how to make it easier; because you hate to see them sad and angry.
Next start working with the child on the "What The Child Needs" steps listed above.
Over the following weeks, be sure to give them praise when they do the things you two talked about and are able to move from one thing to another when asked.
The goal is avoid demoralizing them into thinking that this bad thing is always going to happen to me, and instead build them up to the point they see it as another important skill they have gotten good at.