Can I learn to speak a foreign language without an accent as an adult? The answer is no.
After your early teens, the chance of learning to speak a new language without an accent is vanishingly small. Not only the speech sounds but also the rhythm and intonation will give you away. But many people find a foreign accent intriguing, even sexy. If you like evolutionary explanations, you could think of your accent as a clue that not being from “around here,” you might have genes that would make you a good choice as a mate.
A famous study (Ioup et al. 1994) looked at two women who had learned Arabic in early adulthood. Julie moved to Cairo at age 21 and married an Egyptian. She never studied Arabic formally. However, she was immersed in the language, living with her husband’s family, and made an effort to analyze and understand the grammar. Laura, the other woman in the study, took Arabic courses in college. She too married an Egyptian and lived in Cairo. At the time of the study, Julie had lived in Egypt for 26 years, Laura for 10 years.
Recordings of the two women’s spoken Egyptian, along with recordings of control subjects, were evaluated by 13 teachers of Arabic. Both were rated as native by six of the judges; two other judges rated one woman but not the other as native.
This shows that even exceptional learners with intense, prolonged exposure to the language managed to “pass” as native with only about half the judges.
Outside of spy stories, however, there is not much practical value in sounding “just like a native.” If you communicate effortlessly, with a good vocabulary and a lot of the cultural background that native speakers have, people who know you will stop thinking of you as foreign. Also, as the Egyptian study shows, some people will not be able to detect a slight accent. You may be able to fool some of the people some of the time.
If you feel that you have trouble being understood, specialized courses in accent reduction certainly can help. However, such courses will include a focus on technical aspects of pronunciation, and the results will not come quickly. The main part of acquiring a good accent is to mimic how native speakers sound, which makes many learners uncomfortable as if they were pretending to be somebody they are not. And your accent is in fact a part of your story, your self—do you want to lose it completely?
Ioup, G., Boustagui, E., El Tigi, M., Moselle, M. Re-examining the critical period hypothesis: A case study of successful adult second language acquisition in a naturalistic environment. Studies in Second Language Acquisition 16:1, 1994. pp. 73-98.