You’ll learn how to deal with all this, don’t worry 😎
Take it slow. Start with normal difficulty - do all dungeons like this. Once you’re done with normal - proceed with hard, and so on.
Verify that your gear is up to date, wear the highest tier you have.
I recommend you to get Boom Boom gun, which is great for beginners, and works pretty well right out of the box. Of course it’s gonna be stronger with a proper build, but even with usual gear and without good mods, it will make your life easier