How do I stop confusing the retroflex 'ṭ' and 'ḍ' sounds?

I’ve been recording my progress for months, but when I listen back to my Sinhala sentences, my 'ṭ' (ට) and 'ḍ' (ඩ) sounds still feel too much like English 't' and 'd'. I really want to nail the authentic Sri Lankan pronunciation before my trip to Kandy. Has anyone used Chickytutor.com to get specific feedback on these retroflex sounds, or are there better ways to train my tongue to hit that alveolar ridge properly?