Time Management – How to switch to another topic

I’m trying to plan my day and include time slots for the several topics I’m working on.
However, I’m having a hard time switching from one topic to another.
Actually, what is happening is that it took time and energy and discussions with my brain to start a topic (Topic 1). And then once I’ve started and it’s time to switch to another topic (Topic 2), I have a hard time stopping as I haven’t finished this topic 1. I know that I need to stop and switch because topic 1 is requiring more than 100 hours to be done and I’ve divided it through several days.
What I end up doing is buffering with other things (SCS, housework, administrative work,..) so I can feel “useful” but I don’t do either Topic 1 or Topic 2…
Can you help me with this please?