I was recently invited to go ride a trail with a couple of friends.

I would be lying if I didn't say im a bit freaked out on the what if situations. Has anyone ever flipped over or fell from hitting a rock you didn't notice? Or a thick tree branch?

Don't mind the what-if situations. Determine the technical difficulty of the trail (perhaps with a relatively low speed loop your first time around, so you know what to expect), and ride within your abilities. If your more experienced friends go bombing down a slope, don't be tempted to follow them at their speed unless you're sure you can handle the terrain. I began to follow this guideline after my first accident, and haven't had another one since.

No shame in walking through a section that you find intimidating. Better than getting in over your head and skill level and having no way to safely stop/bail.
"Walk today, ride tomorrow" is a good motto. Spend a little time reading up on some of the very basic skills--it will help a lot.

I've been riding for a little over 2 years and I am still scared to death when riding.

All of the above. Wrecking is part of it. If it is your first time I doubt they are going to throw you onto an advanced trail or they aren't very good friends if they do .

I've been in your situation before. I usually walk stuff I'm not comfortable with unless I'm in a particularly adventurous mood. It's always a good idea to ride only what you think you can.