I Started Invisalign a Week Ago, I Want to Discontinue, Is that Possible?

Not sure why you are having second thoughts.  You really can't see any change after wearing only one aligner.  What are the issues that you are having.  Most people struggle in the beginning and then love wearing them and they are thrilled when they are done with treatment.  Stick with it and it will pay off in the end

If you just starated your Invisalign a week ago you could stop now and your bite would be unchanged.  Any small tooth movement that the first aligners accomplished would revert back and the teeth would be pretty much the way they were before Invisalign.  But why are you having second thoughts?  Are you worried that the goals you've chosen to pursue are not what you want?  Are you having trouble adapting to your aligners?  You should discuss your concerns with your Invisalign provider.  If you honestly present your concerns I doubt he would "fire" you as a patient; on the contrary I bet he would welcome the chance to deal with whatever problem you're having.

When you first get your aligners they will feel foreign to your mouth and you may be tempted to just take them out. Stick with it, wear them the full 22 hours per day and your mouth will adjust and feel comfortable again. I have had my Invisalign for 6 months, and I don't even think about them anymore as I go through my day.

You are not clear as to why you want to stop treatment. You should be noticing a change, isn't that what you hoped to see? Discuss your concerns with your dentist. The treatment plan can be modified thru a mid-course correction, if you've decided on other goals. If you just stop cold right now, your teeth probably will NOT occlude (meet each other) properly.
 

Moving your teeth and the changes (hopefully good) to your smile are a big deal and it is perfectly normal that you might be feeling nervous about the process. People change their mind all the time, so I am sure your dentist will understand.

You can stop treatment at any time, but before deciding whether or not to continue I'd speak with your dentist about your concerns and maybe they can put you more at ease. Keep in mind that your dentist has already paid significant lab fees to Invisalign to have your aligners made, so it is unlikely you'll get a full refund.