I've found in my life that in general I grossly underestimate the amount of time required on any given task/project. I have made conscious efforts to actively double my initial estimate with a project.
Ex: Until recently I would plan 2 Saturday projects (cleaning + hike) and realize that the cleaning project actually took all day or more. We finally have experienced success in planning on taking all day on any Saturday project and anything left over in the afternoon can be bonus time. Over all this has created more successful projects, and less stressful Saturdays.
Is this a common problem for everyone else? Or am I just overly optimistic in my capabilities?
Serious question: How old are you?
I ask because I'm pretty good at estimating how much time a certain task will take me, but my students -- oh, my, time management is the single thing they do worst. When I sit down to work, I make a plan, and I DO IT. When I give them a task -- and I'm talking about even my very best, cream of the crop students -- they dilly-dally about, they arrange their materials, they take a break, they talk a while, they mess with their phones, they work a bit, then discuss, then change the plan halfway through, they come to me for advice, they take another break ... typically they spend 3-4 hours doing what I can do in one hour. It's because they don't work efficiently.
They don't get down to business and work. My older daughter went away to college in this wishy-washy state, but she transformed herself into a working machine. She's a senior now, and
the girl is efficient. She's a serious work horse when she sets her mind to it. She gets stuff done right the first time because she doesn't want to go back and waste time at it a second time.
She's me. My younger child isn't "there yet", but I see her improving.