Is my toothpaste doing more harm than good?

I use Colgate Total Advanced Whitening (or something like that). Well I went to the dentist yesterday for a cleaning and check-up. I've got 8 cavities! I brush my teeth twice a day, floss at night, and use a very expensive fluoride mouthwash. I couldn't believe it! The dental assistant told me that sometimes you get cavities no matter what you do. I don't think that's true. But I don't want yellow teeth but the more I think about it I don't want anymore cavities either! Does regular toothpaste (anything but whitening) still keep your teeth from staining? They just make so many toothpastes that are for gum protection, enamel strengthening, and all that stuff. I'm so confused as to what to buy! I only use Colgate though. Can you give me some ideas as to what kind to get? Also I'm going to start brushing 3 times a day with a toothbrush they gave me (its manual). My toothbrush I've been using is electric and I don't think it was doing a very good job. I'm so ashamed because all my life I'll have fillings and half my mouth will have fake stuff in it! When I was younger I never brushed my teeth and was a heavy soda drinker. I don't drink soda anymore thankfully. But by looking at my teeth you wouldn't believe that I have 8 cavities. I guess they must be on the back of my teeth. My teeth never hurt and they're not sensitive. I just switched dentists and my new one is going to have to redo one of my fillings that my previous dentist did because its not right. So basically my other dentist sucked. Well anymore advice you have I would really appreciate! Thanks :)