Re the disorganisation and memory thing:- I find I have to be able to SEE my to do lists to get through and remember everything.
One method is to have an open A4 book on your desk divided into squares for each hour of the working day, or whatever division makes sense for you. Each task is written down in one square with other information you need for it, e.g.phone numbers etc. As soon as you complete it, you draw a line through the square in a different colour.
Try tackling your inbox at set times. I find a short time a couple of times a week spent deleting and sorting into sub-folders keeps me on top of things.
Clothes: don't change your style. If heels aren't you, there's no need to wear them. You can still be smart. Choose things you can mix and match and also update periodically. I do this-one of my favourite types of work outfit is wide, wool flannel trousers, blouse, long cardigan and loafers. I have had versions of this outfit in my wardrobe since I was 18.
Try eg short tweedy jacket or cardigan, plain white, cream or pink T-shirt (I like Uniqlo's) and plain black, navy or grey wide trousers or A-line skirt, worn with tasselled or penny loafers.
If there is anything distinctive you genuinely like-brooches, bracelets, scarves-treat yourself to a couple of nice ones and enjoy wearing them.
Handbags-no need to play the whole how pricey is my handbag game unless you really want to. The Internet is full of lovely moderately priced bags (including leather backpacks) , usually from Italy or Spain, by makes you've never heard of if you do want to get a nice one.
Nerves: admit it, especially when you start, at least to the friendlier colleagues. And don't forget to show people that you like them.
Finally, all the things you mention: lack of confidence, nerves, anxiety, poor memory to point of forgetting tasks completely, not getting instructions, disorganisation, apply to me too. I was recently diagnosed with ADHD. So, while it is probably your anxiety hampering you, there could be a cognitive issue. Worth thinking about.
The ways to deal with this are: don't multi-task. Do one thing, and then the next thing. Work your smart phone to death with lists, notes, reminders, alarms. I need an alarm to tell me to go to meetings etc! Write stuff down while you are speaking to people, not later . Don't put stuff off-you won't remember half of what you need.
There are a lot of self-help books about this. The one that really helped me is this one: