I know my sister in particular laughs at me, but I start packing a couple of weeks in advance so I can take my time and make sure that I have everything I need. The only things I've ever forgotten are the last minute things like a hair brush or the like which is easily replaced. Other things generally would not be a huge issue - I'm not really into pools so a swimsuit wouldn't kill me if I left it, my clothes are generally mix and match, and I have enough underwear that it's the first thing I pack so I know that's in there.
One thing I've learned being female is take more of the stuff for that time of the month than you would normally need. Especially when traveling in Europe. I thought I'd packed enough for my Med/Venice cruise last summer, but my body had other ideas, and I had to restock in Venice. I found a store that had some and went to get them (note: they are different in Europe in that they don't have applicators (just like OB here) - it didn't bother me, but it might some people) - I bought my box, and the guy behind me bought 2 packs of cigarettes for pretty much the same price. So at least in Venice you can do something totally unhealthy for less than it costs to be female each month!
You aren't alone, my wife got us doing this for our cruises and we even have a check off sheet to make sure we have everything packed, down to drivers licenses and passports. I took to the idea and do the same for my fishing trips where having the proper gear packed is very important

Saying that I did fail once on a land trip to pack boxers, thankfully there was a Walmart nearby. Have never made that mistake again though lol.