Here is an excerpt from Ann Cook's ebook, American Accent Training. I use it every time people get discouraged in my accent training classes. (I've done a lot of training in the outsourcing industry and Anne Cook is a classic resource. For a whole decade, many companies thought that you have to really sound American if you were going to work the phones. Thank goodness that's no longer true these days.)
Why Is My Accent So Bad?
Learners can be seriously hampered by a negative outlook, so I'll address this very important point early. First, your accent is not "bad," it is nonstandard to the American ear. There is a joke that goes: What do you call a person who can speak three languages? Tri-lingual. What do you call a person who can speak two languages? Bilingual. What do you call a person who can only speak one language? American.
Every language is equally valid or good, so every accent is good. The average American, however, truly does have a hard time understanding nonstandard accent. George Bernard Shaw said that the English and Americans are two people divided by the same language!
Some students learn to over-pronounce English because they naturally want to say the word as it is written. Too often an English teacher may allow this, perhaps thinking that colloquial American English is unsophisticated, unrefined or even incorrect. Not so at all! Just as you don't say the T in listen, the TT in better is pronounced D, bedder. Any other pronunciation will sound foreign, strange, wrong or different to a native speaker.