(frequent passenger and former nervous flyer speaking...)
If you travel to Stansted by road your journey will be more dangerous than your flight to Ljubljana. Air travel is extremely safe.
The plane will most likely be an Airbus 319. A modern aircraft with an excellent safety record. Airliners are built to withstand a great deal of turbulence - and if the weather en route or at your destination is forecast to exceed safe limits, your flight would be cancelled or diverted.
If turbulence upsets you, choose a seat in the centre of the plane (over the wings) - that's where the smoothest ride is. Some people prefer a window seat so they can watch the horizon - that can be reassuring if you are nervous, and also help if you tend to get airsick.
For what it's worth, my flight to and from Ljubljana was an extremely smooth one.
And if you are struggling to relax en route, think about your desintation and remember how lucky you are to be travelling to such a beautiful country
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