In addition, my boyfriend surprised me a few times when I was lying in bed reading a book by jumping on me from behind. I shouted at him because I didn't expect it, and even though I was the one sitting quietly not bothering anyone, I get attacked by Lucy.
Did Lucy see that he jumped on you from behind or did she only hear you? If the second was the case, she naturally tried to defend him. But even if she did see it, she did not thought that he attacked you. Some Cats do attacking each other just for fun, so it is a known principle. And they are good in reading the body language of humans. She had seen that your boyfriend came to you full of anticipation for his attack and without any aggression towards you. She recognized that it was just a game. But you were a bit aggressive towards him when you reacted, right? So she defended him.
There is one main thing between you and this cat. And this is your boyfriend. Lucy was first, right? So he was caring just for her when he was at home. And now you are there, too. And your boyfriend is giving you attention. Probably more than Lucy, right? And this is correct and nothing should change. Lucy has to learn that it is ok that you exist and that you belong to him.
So the first lesson is that you have to be self-confident but calm. You wrote that you have the feeling that she does not like loud noises. That is possibly true. So try not to make loud noises.
The second thing you want her to do is loving you. One thing to know is that trying to pet her all the time or even looking at her will not help you with this. On the contrary it is best to start to ignore her. Don't watch her and if she comes and tries to attract attention, don't react much. You can look at her for a short moment, but you have to be the one who ends up with it. Then you can start to ask her for attention and she likely will accept that. Try to play with her or gave her some treats. There are 3 rules:
- You start the game or giving treats. Never give her what she wants if she starts to attack you or begs for something.
- You end the game or giving treats. You have to stop it before she loses interest.
- Stop the game or giving threats immediatly when she starts to be aggressive towards you.
The key is to always stay calm even if the cat does something wrong. If you try to discipline her it is best to either stop doing positive things with her like giving treats or playing with her. Do this immediatly in the second she started behaving in a way you don't like. Or, if there is nothing you can stop, show her you don't like what she is doing. For example make a hissing noise and stare at her. If this is not sufficient, you can push her away. You are stronger, so do not hit her but you can shift her. Always start with a soft reaction and increase the pressure until she stops. Raise your voice and shout at her just in case nothing else works.
A good tip for rewarding an overwight cat is to lock the food so that she can't eat whenever she wants. Then portion the food so that you know how much you can feed her a day. And then divide this portion into smaller parts you can give as treats or as regular feedings. Feed her as often as possible if she was used to having the food always around, so that it is easier for her to rearrange.
At first it will be really difficult to find a situation you can feed her, because she possibly is aggressive really often, but you have to wait until she does not attack you or is not begging. Be patient. This time will pass. You can try to put her in another room just for a minute, if she does not calm down, and then free her and give her some food, before she has the chance to attack you. If she was faster, put her back into the room, wait a minute and try it again.
And last but not least it is really important that your boyfriend supports you in disciplining the cats. It won't work so well if he does not support you. The cats will recognize it and he has more authority than you at this point of time. Speaking of support, it may be a good idea if he is defending you, when Lucy starts to attack you. It shows her that he is at your side and helps her to find her new position as second after you.