The specific problem is that American intervocalic /t/ and /d/ is very similar to Spanish /ɾ/. But if they don't get it right it's not perceived as the right phoneme. The Spanish /t/ is more dental and the undervocalic /d/ is more of a [ð] but they will sound correct in English.

> An easy way to improve intonation and fluency is to imitate a native speaker.

There are lots of variations in English pronunciation. Singaporean, Australian or Scottish native speakers do sound very differently. I don't know to what extent they benefit from adjusting their accent if working in a different English speaking country to match the local dialect.

Also, as a non-native speaker I wonder if it's worth practicing my accent considering that everybody has a different accent anyway. Rather than trying to mimic a north american accent (which I'll never be able to do anyway), I'd be more interested to identify and fix the major issues in my prononciation.