Welcome to the forum and hissing is exactly what they do when they are scared so I don't think it's a 'harsh' word for it. Now, the word that needs to register from the last sentence is 'scared'! Because that is the reason why you are not making any advances in her behavior. You need to show her that there is absolutely nothing to fear from you, that you will never present a threat to her and that only good things come from you. Cockatiels are aviary birds [not companion like senegals, cockatoos, macaws, caiques, Meyers, redbellies, etc] and yours was obviously parent-raised [versus handfed by a human]. Her nature and the way she was raised are the reason why she is so scared when you approach her so you need to get her used to your presence without stepping on her 'boundaries'. Let me explain what I mean by giving you pointers.
First of all, she needs to feel completely safe in her cage so it needs to be high up [her roosting perch needs to be at your eye level when you are standing up because nothing scares a little prey bird more than a big looming presence over it]. You also need to put it against a wall or, if this is not possible, drape a material in the back making kind of like a 'shallow' cave so the material will cover the entire back, a bit of the 'roof' and a bit on either side. This will also make her feel safer because she instinctively knows that 'walls' prevent a predator from attacking from that side.
Then you have to establish a strict schedule and daily routine - NOTHING reduces stress in captive animals more than a strict routine allowing the animal to foresee and expect different actions at different times of the day.
Now, to get her used to you. You already have the right idea in that you need to spend as much time as possible with her. At the beginning. the only thing you do is sit at 'her safe distance' for as long as you can while you talk/sing/whistle to her WITHOUT LOOKING STRAIGHT AT HER [staring is a predator trait so look at her from the corner of your eye until she is used to you and no longer fears you]. Her 'safe distance' is determined by her behavior when you approach her cage. Don't walk in a straight line at her, just amble around, as if your goal was not her cage but always keep on looking at her from the corner of your eye and, as soon as you see her moving in the opposite direction from where you are coming, STOP, take a single step back and MARK the spot because that is her safe distance and where you should sit to spend, at least, two hours a day. Mind you, it doesn't mean that you should sit in the middle of a room doing nothing for two hours
Make it so it's next to a table or where a sofa is so you can watch TV, read a book, play video games, whatever. Always talk saying her name and praising [they might not understand the words at the beginning but they do understand a tone of voice] and, every now and then, give her a treat [a sunflower seed or an almond sliver will work] BUT, if she doesn't take it from your fingers [and she won't at the beginning], just leave it there for her to get after you move away. This is not a reward or a bribe, it's a gift, a token of friendship.
You will have to clean her cage, feed and water her which she will not like so always use the same words to let her know what is coming. I say "Excuse me - excuse me - excuse me" to all my birds -even the ones that are not parrots and I am not trying to tame or win over- but you can, of course, choose your own phrase for it. The idea is for her to learn that when you say this words, you will go into her cage but that you will not try to touch her or ask her for anything.
Once you are able to get close to her cage without her hissing at you or backing off, you can start target training but, in order for this to work, you will have to make sure she gets no high protein food all the time because, if you do, she will have no incentive to do anything for a high protein reward. Besides, it's very unhealthy for them to be free-fed protein food, it messes up their liver and kidneys. I recommend gloop with a different leafy green every day from breakfast and a good quality cockatiel seed mix [maybe one safflower based?] for dinner with a good quality multivitamin/mineral supplement a couple of times a week and a cuttlebone or a mineral block in her cage all the time. You can also give her fruits and veggies but, in my personal experience, the only veggies they eat are the ones in the gloop and they hardly ever eat fruits but they do LOVE their greens.
One more thing, be VERY careful with her light schedule because female tiels are VERY prone to chronic laying and eggbinding which kills them if it's not treated immediately.