Hey happymunky I was in the same boat as you when I started with the Mr. Beer kit. It's a good kit to start out with, but what you probably don't know - I didn't, and it sucked - is that the Mr. Beer directions skip and leave out a lot of terms and techniques that veteran brewers use all the time. This was absolutely frustrating. It's like the b*stard way to make beer when compared to what everyone else on HBT is using.
The MB directions say add sugar to your bottles at the bottling stage to add carbonation, while most users here who use much more elaborate kits brewing larger batches boil a few cups of sugar and water, add to a bucket, then pour their fermented beer on top of it, then fill the bottles up from the bucket with the sugar already in the beer.
I had the hardest time starting out with Mr. Beer because the directions were SO simple, but the folks here used so much added equipment and technique their answers went way over my head when I asked questions. So if you feel it happening to you, slow down. There's nothing to gain by going fast and getting frustrated.
Since the Mr. Beer kit mentions nothing about hydrometers, wort chillers, boiling your sugar for bottling, etc. you probably don't know what any of that means or what they do because I didn't (MB directions mention none of that). I gauged the right time to bottle by simply tasting once every seven days. In simple terms, when that sweetness is gone, wait 5 more days and bottle. The kit says do not bottle before 7 days, best results if bottled after 14 days. Typically, I let things ferment in the Mr. Beer fermenter about 21 days. Your problems arose simply because you rushed it a little much.
Take your baby steps for the first few brews, THEN start learning more about hydrometers, wort chillers, boiling your sugar, etc. By that stage you'll be ready to dive in and get bigger and better equipment. For me, it took about 30 gallons of Mr. Beer to get to that stage. It'll be overwhelming if you try to absorb too much from the experts, but once you know more, you'll start to benefit greatly from what they know cuz you'll be operating at "almost" the same page as them. This is the stage I'm at, I've moved on from MB but I think I'm benefitting much more from the good folks here at HBT then I did when I first started with the simple Mr. Beer 2.5 gallon system.
Get
THIS BOOK after you've made 5 brews. It'll help. And it'll make a little more since once you've got a few batches under your belt.