I'm 13 and scared of buying?

I’m an aspiring illustrator and there are a lot of different tools I want to buy, like derwent/ prismacolor/ sakura etc. My mum gave me £15 to spend on the cheaper materials. I went to the shop and found myself analyzing every art item. I looked at the price, compared the quality to others, which ones are worth the price etc etc. When I finally found something I liked (a derwent watercolour pen -£5) I started talking myself out of buying it and kept saying that it wasn’t worth the price. I did that with many items, even a notebook that cost £1. When I tried to force myself to buy something that I knew I really needed in my work I was close to crying. In the end I didn’t buy anything. It’s not the first time this happened. Every time my parents buy me a present or when they buy me something at the shop I feel extreme guilt and try to talk the out of it. We are a poor and indebted family so I’m used to always calculating every penny spent on my things and am always guilty about costing my parents too much. I always scold myself. Is that normal? Anyone else in a similar situation?