While every case is different, and there can be no generalizations about so personal an experience, recent studies have shown that there are few long-term psychological effects for women who choose to have an abortion, unless they have pre-existing emotional distress or are terminating a wanted pregnancy. For example, in 2008, a study conducted at John Hopkins University reviewed 21 studies involving more than 150,000 women, and determined that "The best quality studies indicate no significant differences in long-term mental health between women in the United States who choose to terminate a pregnancy and those who do not". Here's a link to that site:
The American Psychological Association has also convened several studies, the most recent in 2007. They concluded that "Although there may be sensations of regret, sadness, or guilt, the weight of the evidence from scientific studies indicates that legal abortion of an unwanted pregnancy in the first trimester does not pose a psychological hazard for most women." However they did note that "women who are terminating pregnancies that are wanted and personally meaningful, who lack support from their partner or parents for the abortion, or who have more conflicting feelings or are less sure of their decision before hand may be a relatively higher risk for negative consequences." Here is the link to that:
On a personal note, my mother chose to have an abortion, and went on to have three children with no adverse psychological effects. To this day, she tells me that she does not regret her decision. But again, there can be no generalizing, and I'm sure each of us can quote studies all day that support both viewpoints. It's really something that can only be determined on an individual basis.