I can only do one task deeply at a time but while doing that, I will think and worry about other tasks that need to be done. This stops me from what I am working on, leaving it half way and starting something else. When I reach a point when two or more tasks compete for my attention (or someone drops something on my plate) I feel locked up.
The best thing I've found is to capture tasks as soon as I get them and try to ignore things until my current task is done. Otherwise, i drift of into mindless scrolling