I suffer from "programmer's optimism": I invariably underestimate how long a project is going to take.

And so, after 25 years experience, I've come up with a pretty reliable algorithm for computing schedule estimates:

1. Take the "instinctive" time I think I can do it.
2. Multiply it by 3.

I've been very surprised at how accurate that simple algorithm has been. Whether its just a week long project (which my gut always tells me will take "2 days, tops"), or a 12 month project (which my gut always tells me is a 4-5 month effort).

YMMV, of course. Your multiplier may be different. And if its going to be a team effort, that may effect your multipler as well (tho, inexplicably, I've found the 3x factor seems to work in that case too)

Presumably, you've concluded your estimation skills are flawed based on prior experience...so take the estimates you made in those cases and compare to how long the project actually took. You may be amazed to find the same result I did: a simple multiplier, regardless the duration/size of the project.