A study by Joanne Wood and John Lee of the University of Waterloo and Elaine Perunovic of the University of New Brunswick asked participants to say "I am a lovable person" to themselves sixteen times in four minutes. People with low self-esteem actually felt worse about themselves after the four minutes were up. Time's John Cloud offers several explanations for this failure of a commonly recommended mood-lifting technique. For one thing, he writes, "when people hear something they don't believe, they are not only often skeptical but adhere even more strongly to their original position." So if you think you're terrible, but tell yourself you're lovable, you may end up feeling even more terrible as a kind of rebellion.
A variant on this explanation is the finding that, "when people get feedback that they believe is overly positive, they actually feel worse, not better." Most people, even if they have normal self-esteem, have probably walked away from excessive praise feeling weirded out, or wondering if the praiser really knows them very well. And blanket affirmations like "I am lovable" may be the self-talk version of excessive praise.
What Cloud doesn't address is how vague — and kind of depressing — the statement "I am lovable" is. Being lovable doesn't mean you're actually loved, or kind or interesting or smart or happy. Most people, in fact, are lovable at least to someone, and the word doesn't say anything about a person's actual being. Telling yourself that it is possible that someone could love you actually seems like setting the bar pretty low.
Compliments from other people tend to carry more weight the more specific they are. It's easy to brush off "you're great," less easy to ignore "what's great about you is how you approach new situations with such confidence." And while it's always harder to believe your own compliments than other people's, it might help to start with compliments that don't suck. We'd like to see a study where people were asked to come up with something specific that they authentically liked about themselves, and then repeat that bunch of times. They might end up feeling more than just "lovable."