I used to be the same way OP. What made a difference for me was realizing that lying is SO MUCH HARDER than telling the truth. The stress of covering up insignificant lies took so much energy and stopped me from being my authentic self in relationships.
Now, instead of lying, I simply try to live a life that I don't feel a need to lie about. That doesn't mean I don't sometimes waste hours watching shitty TV, smoking weed, and filling my bedsheets with chip crumbs, but it turns out no one really cares about those things. But when I'm debating whether or not I should do something, I ask myself if I'd be comfortable admitting doing it to my boyfriend/best friend/etc. And if the answer is no, I don't do it (for the most part. No one is perfect.)
The next time you find yourself telling him you're eating a salad when you're really putting down an entire pizza, try saying "I don't know why I said that. That was stupid, I'm totally eating a greasy pizza right now." It'll probably feel good, and you won't get in trouble.