You need to learn to distinguish between sin and your fallen nature. It's not a sin to have bad thoughts or feelings unless you consent to them or entertain them.
You also need to learn to distinguish between venial and mortal sins. For a sin to be mortal it has to be (1) something very serious (2) whose sinfulness you have full knowledge of (3) and fully consent to.
How do you know if something is very serious? Here the Ten Commandments are helpful. There are a number of printed and online examinations of conscience which you can use as a guide, such as this one. It's also okay to get help from the priest; he can tell you if something is mortal or not.
We are required to confess all mortal sins in kind and number. We are not required, but it is a helpful practice, to confess venial sins also, but unless your confessor or spiritual director advises it, I would avoid trying to make an exhaustive list of all venial sins, because this can become an occasion for scrupulosity.
You're right about sins of omission. If you don't do something you should, this can be a sin. (It can also be an imperfection, depending on the situation.)
You get better at examining your conscience if you do it each night.