Ruby Vee

90 year olds die.  Especially very sick 90 year olds.  I doubt very much it had anything to do with you administering meds through his G-tube.  (Even if they were the wrong meds, unless you made an egregious error. As for egregious errors -- you are unlikely to make one because you seem to be very vigilant.)
Confidence comes with experience -- I used to be hyper- vigilant just like you.  I remember once being so worried about whether or not I had actually changed the dose of the heparin drip as ordered that I drove back to the hospital at 1am, sneaked up to the unit the back way and peeked into the patient's room at the drip to make sure that I had.  I had.  Did that cure me of my anxiety?  Not immediately, but as I became more confident in my knowledge base and in my system of checking myself for errors, the anxiety and hyper-vigilance ebbed.  
Devise a system for yourself so that you can relax, knowing you've checked all the things you're supposed to check.  That still won't prevent you from EVER making an error.  We're human.  We all make errors.  There's even a thread or two about that here -- I encourage you to look for them.  But with a system, you minimize the chance of errors, maximize the chance that you will catch an error before it gets to the patient and it helps you to stop driving yourself crazy about potential errors.  Then you can concentrate on the big picture, which you may be missing now because you're so worried about making an error.  I missed a number of "big pictures" because I was so worried about the possibility of making errors.  A system for taking report, giving meds, assessing patients, etc. helped enormously.  
Good luck.  You seem like you'll be a great nurse one day.

Employers don't care where you went to school as long as it is an accredited, brick and mortar school. What they're interested in is your LICENSE. If both schools have similar NCLEX pass rates, choose the one that is less expensive/closer to home.
Going to UMass isn't going to give you an "edge" over other new grads. If you want to go to UMass and can afford it, that's where you should go. But don't choose it thinking that it's going to somehow give you an advantage over other new grads.
Student loans are a millstone around your neck until you pay them off. It is feasible to pay them off on a nurse's salary, but like everything else you make tradeoffs. I wasn't able to buy a house until after I'd paid off my student loans, and the debt made me decide against doing some things I would otherwise have loved to do. Borrow as little as you can get away with and repay them as quickly as possible.
Getting a BSN is a good plan, and I'm glad I started with a BSN. But I have dear friends who started with diplomas or ADNs and then got the BSN with tuition reimbursement.

The first year of nursing is miserable -- if you haven't already done so, check out some of the threads on the "First Year After Licensure" forum. That said, six months is way too early to be giving up. Yes, you've had some rough shifts and you're not meshing well with your colleagues. That's pretty normal. Crying at work is somewhat normal, too, only most of us manage to wait until we get to the bathroom to give in to it. Your colleague was right, though -- your patients needed you. And you recognized that right away.
I don't think a new job is the answer. The first year is rough, and a new job will just give you all the same issues with different faces and supplies in places you've never seen before. The answer is to stick it out for a whole year. Around the one year, something "clicks" for most of us (although I was slow and it took me longer) and we start to feel like we've got this -- at least some of the time. We start to be more confident, to know what we don't know and to know where to find answers. Most of us are surprised to find we actually LIKE the jobs we thought we hated six months in. For those of us who still hate our jobs, our colleagues or our specialties, it's much easier to find a new job after a full year of experience than it is after six months.

Were it not for the fact that I'm THIIISSS close to retirement and my pension is at my current hospital, yes I would quit over going to all white uniforms. It's just one more way for management to put a collective foot on nursing's collective neck.