I have one nostril completely clogged especially when I lay down, makes sleeping rather difficult. I've already tried Flonase and peppermint last night but to no avail.

Try various decongestants on the market, including those in pill form.
Also try anti-histamines.
Read all the labels so you are trying different main ingredients but not duplicating products.
When I have one nostril clogged, the other one hurts from doing all the work. I stuff wet tissue in both nostrils.
Don't sleep with air conditioning on. Try a fan in a window aimed slightly away from you.
If you live in the south and must have AC, at the very least clean the filter and make sure water isn't staying in the shallow tray at the bottom, growing mold. And turn it down at night, sleeping under just a sheet or no cover at all.

Saline nasal spray works well and there is no limitation on how many times you use it because it is not medicated.

Don't sleep on a flat pillow, use two to elevate your head when sleeping. I have been through this before and found out after all these years I was allergic to my Down filled duvet.