I guess one of the mistakes you made was planting the same crops in 2 consecutive years.
The crops planted in the previous years would have used nutrients from the soil. This could have contributed to the poor results you had this year. Your current crops are deprived of nutrients.
Another consequence could be pest and desease. The reason why rotate crops (or don't plant them twice in as many years) isto avoid pest and desease.
Some pest lay eggs in the soil. These hatch when your vegetables are begining to mature. The young pests then feed on your crops. But what happens when you rotate the crops.
Well if a pest feeds mainly on tomatoes but is faced with peas when it exits the eggs then it either starves to death or finds another garden to atack.
So maybe you can begin rotating crops and become more structured in your approach to gardening.
You can check out the following site for more information:
Vegetable Garden, Vegetable Gardening Tips and Planting a Garden