Realistic Time Goals

I usually keep track of how much time certain tasks will take me. But when planning, I am overly optimistic, and think that *maybe* instead of getting, say 30 inbox items done, I’ll try to do 50.

If I don’t challenge myself, I think I won’t stretch/grow, but the end result is I never have an accurate idea of what time things actually need and I always feel behind. How to find the balance between challenging myself and seeing what I can do vs being realistic?