I have the same problem. I use to be so meticulous and I let myself go. I too think I ought to spend money on anything except my appearance.
I am starting simple. I wear clear polish and lip gloss. I use creams and lotions to keep my skin nice and soft. I am working on my feet now that I can reach them. I recently went to the thrift shop to buy better fitting clothes and forced myself to throw away the well worn fat clothes. The other day I saw how ratty and huge an old favourite shirt had become that when I took a shower that day, I took the shirt off and threw it in the trash. This is something I am making myself do, even if it means I have a tiny wardrobe. Better fit is becoming more important to me.
The next step for me is to pick colours and styles that I like, even if it remains being ultra cheap clothing, because I don't want to invest in transitional clothing. Then I want to add soft colours, perhaps a soft pink lipstick and fingernail polish.
It seems to be easier and less threatening to do this slowly. As weight comes off, the inspiration starts to come back. I started a collection of pictures I keep in a folder called "I would wear that". Any item of clothing or shoes that catches my eye, I copy into that folder. I am hoping one day I will be able to get a better sense of how I want to dress, a sort of signature style that is just me.