That's like preparing the dinner menu while eating lunch. It can be done - by being focused on ONE idea at a time. (And you shouldn't talk while eating anyway. Swallow first.)
We sewers all struggle with motivation.
Early in my historical sewing days (about 10 years ago) I didn't realize how much TIME it actually took to complete a gown. Oh! the last minute sewing and VERY late nights I'd put in before an event. And after each event was over and the costume was put away I'd vow not to kill myself like that again.
I set out, determined to overcome this lazy procrastination of letting the fabric sit untouched while I went strolling through my Drool Books - musing over that dream gown that I'd make "someday".
Over the years I've cut down on the Panic Sewing but it's still not entirely gone. But it has become extremely better.
To keep yourself out of the Lazy Sewer's Box and energized on what you are working on NOW is to make it your goal to NOT lose sleep over the project. At what point does it become Not Fun when you lose sleep and get stressed over some clothing?
Set your deadlines and make your lists. Focus on baby steps if you have to. Sooner or not you'll have completed that project (DAYS before the event) and you'll be ready to start the next one.
But What If You Get Bored?
Have you cried at the frustration that your man's "must sew" waistcoat and pants are taking WAY too long and you're dying to start cutting the pretty lavender that will become your delicate gown?
Have you sewn enough seeds beads yet?
Am I done with all this white undergarment fabric yet??
My suggestion for boredom is to prevent it with VARIETY. There's no reason you can't work on multiple projects at once. (I'm currently working on my 1844 Summer Dress (cut out) and my 1770s Pink Stays (bone channels sewn) - pictures to come.)
However, I must worn you about dual projects - they can drag you down to the point of being a Panic Sewer. Do not let this happen.
To avoid this, make your (detailed) lists of steps you need to do to complete a project. Do this for the next two or three pieces you want or need to make. Then schedule your sewing time to accomplish items from one list. For your sewing sessions, focus on just one item or one step of a garment.
I rarely work on two projects in one day unless I have a large chunk of sewing time. Even then, I work one hour on the dress then the afternoon on the stays. And I'm not bored. If I don't like cutting 325" of bias for piping I'll go hand sew eyelets. But I know bit by bit they'll both get done. And when one is finished I'll add in that third project (a 1780s matching caraco and petticoat) to my to-do list.
Do you think these two ideas will help you stay on top of things? What have you found to be your best motivation on a sewing project?