I actually went through something very similar recently. I'm about to graduate as a bio student and after starting philosophy I found that all my previous ideals were being uprooted and tossed around and I had very little to believe in anymore. Philosophy can look at times as though it's really good at tearing apart ideas and very poor at building back stronger ones. But as I went more into it I learned that it isn't all hopeless, there are plenty of coherent world views that will adhere roughly to things you thought before. For example, even if I haven't decided on a moral theory that I completely agree with yet, doesn't mean I don't know how to be moral on a daily basis.
It's hard to explain but when you are depressed, the nihilistic ideas about how it all is meaningless and stuff seem to be the only thing that could ever be true. Over time and when you are feeling better it stops being such a big deal. It becomes kind of a background truth that you of course accept but you can live with and build on.
From a biological standpoint I don't think we need to worry about immortality yet. All those 'anti aging' drugs are not going to work. Things are not going to get out of control any time soon. And as for nukes, we have survived the worst of it already during the cold war.
I hope this was somewhat relevant for you. I totally agree with what the others have said: diet, exercise, and especially sleep play a large role. Getting your sleep hygiene right is really important. Other than that, if you have a lot of anxiety or depression try to get help if you can, it won't go away on its own (at least not easily). Best of luck to you