Hi,
Im sorry, I quite down at the moment and lack energy, I read through all your post, but don't have the energy to read all reponses, sorry, so I might repeat what they say.
Firstly, I am sorry to hear you are feeling this way, it not nice. Secondly, I can relate to most of what you are saying, I have been depressed on and of from about the age of 3, and when I was late teens early twenties i went through a particulularly rough patch. I was on the surface happy, but inside...no I wasnt. Like you, I was going a degree, and I felt like I was letting every one down if I would tell then that I just wasn't happy.
I would like to say, you are by no means letting anyone down. You sound like a concientious person...you talk about being having bad thoughts towards other and being irritable, yet you say that you feel guilty. You sound like a nice person to me.
You also mention you had good parents, you sister also sounded supportive. Her advice, to go to the doctors is sensible. They should listen to you sensible. Describe your symptoms. If you have to take medication, that isnt a problem really. You might start taking it, and in a months time, feel fine again. You havn't described any particular uses of sorts that may lead to depression, but maby some kind of counciling or CBT would be useful. If the doctor does offer it, I give it a go.
I think you should see the doctor, as you don't deserve to live this way, so if you can shift this depression you can enjoy things more and worry less.
Take care,
Steve