I believe that you could greatly improve your pronunciation if you took a pronunciation class with a native speaker. If that is not possible, I highly recommend a book entitled "Realistically Speaking" by Planaria Price. Ms. Price taught for more than 30 years at an outstanding ESL school for adults in Los Angeles, Calfornia. Her students say that after 6 - 12 months their pronunciation truly improved because she taught them how to put their tongues in the right place in order to pronounce the sounds of the alphabet correctly. Her book gives exact instructions along with illustrations. On the Web, you can find out more details about her book and how to get a copy.

Try and break the words you are finding difficult to pronounce into little parts. When you can pronounce each part well then add them all together and start practicing pronouncing the word as a whole. Also saying the word very slowly helps, and making sure you are using the every single part of your mouth. Forvo.com is also a very good website. Check it out.