For b you might want to write down what is it exectly you need that day. Women often just go shopping and whatever they see is nice they buy, even if its cost alot of money. You don't want to do that.
Lets say you want to buy a sweater, you need to write down 2 stores you are going to go that you know sells nice sweaters. Then you need to figure out how much money you are willing to spend eg somewhere between $35-$75 (or whatever an average cost for a nice sweater where you come from). When you go to the store, go directly to the sweater area and focus your search there, ignore the urge to buy something else in the store. If a salesperson asks you if you need any help just tell them nicely" no thanks I am just looking". Find 2-3 sweaters you like and go to the change room to try them on. If you like them then buy them, if you think you don't really like them, then don't buy them, leave them in the change room. You should be done in 15-30 mins tops per store.
As for C, don't think about other people but what you need. Its often true that people will treat you nicer and will give you more respect, if you dress yourself in good threads as well. Try to be more relaxed when shopping and not someone who needs to get out now because of a ticking time bomb.
It is often true that we project our own fears onto other people and therefore they treat us accordingly.
Nobody really cares what you buy anyway, people have much more important things to do in their life then worry about you, and if you feel you are being snickered at then know that those people are real losers and are only doing that to make themselves feel better about their own shortcoming.
Also low self esteem should tell you that you need to increase your confidence and live a happy life. There are other articles on this site to help you.