Keeping tabs on sin for confession

I am finding, a whole week into my Catholicism, that I am going to appreciate going to frequent confession. I'm still trying to get a hang of this Mortal/ Venial sin thing and which ones need confessing before taking the sacrament and which ones are ok to keep until confession because they are only venial.

Last week, when I was thinking of my sins, every day 2-3 times a day I would go over my "mental list" of sins so that I would make sure that I wouldn't forget anything when I confessed. When they dealt with bad thoughts, such as anger towards someone, I found I would relive that anger over and over each time I thought of the sin. I just want to get it out of my head. I don't think I need to go to confession multiple times a week, but I definitely need to go to confession every week or two.

I already have one in my head that I keep playing over and thinking, was that a mortal sin? I'll go ahead and say it, because I'm sure the same thing has happened to everyone else. My husband and I were at a clothing store buying a skirt for me. I went to a second clothing store and found a better skirt (it was longer, and I didn't need a slip with it), so I went back to the first store to return the skirt. When I purchased the first skirt, I used a 30% off coupon. But when I got my refund, I got the full amount of the purchase. I feel like I stole that $3 difference. Now, in my defense, they were having serious problems with registers and there were 9 people behind me who had been waiting at least 10-20 minutes to ring up. To bring it to the cashier's attention would have meant having the people behind me wait another 5 minutes until the transaction was rerung, plus it may have taken even longer since the registers were acting so badly.

Also, I lied to them when I returned the skirt. I told them I was unable to find a slip to go with it. I just didn't have the courage to tell them that I found a better skirt at their competitor. I feel guilty about it and feel the need to confess it. But is that a mortal sin or a venial sin? I know the lie was a real lie, because I sat there and tried to figure out what I was going to tell them before I went into the store to justify returing a skirt I had purchased 30 minutes ago.

I have a couple of other small sins similar to the one above that, while not terrible, I'm having to keep a mental tally of, and it's hard to remember them all. I'd rather just confess them and learn from them and get on with my life, you know? And I want to make sure that I don't take the eucharist when I'm not worthy.

I've found multiple web sights that help a bit, but I still worry that I might be too sinful to take the eucharist. I pray for forgiveness and bless myself with holy water but I just don't want to offend God.