I can't figure out how to speak fast and smoothly some conosonant combinations. Could you native speakers please tell me how your tongues move when you try to speak these?

- Alveolar (/t/, /d/, /n/, /ɾ/ (as in better), /s/, /z/, /ɹ/ (usually used as /r/), /ts/, /dz/, /tʃ/ or /dʒ/) or postalveolar (/ʃ/ or /ʒ/) consonant(s) + dental consonants (/θ/ or //). The dentals make me push my tounge forth too much, and it's kinda hard to speak them after any other sounds, especially the alveolars and postalveolars, as in:
+ And then
+ Nothing
+ Ninth
It's hard to produce a dental after a dental, too, as in "The third"

- Alveolar combinations: I always admire to know how the native speakers speak many sounds in a row such as /sts/ or /ndz/, as in "tests" or "ends". I have to replace the /t/ with a short /ɪ/ to ease it, so "test" could be pronounce /tesɪs/, which is how American black people deal with these sounds.

- An alveolar flap (as in better), in General American) would be such a problem if it's preceded by an /n/, as in "don't I". I learned from Wikipedia that it would be nasalized. Is that true?

I wouldn't ask you to analyze the sounds or make me a diagram or something like that. Just tell me how you feel, how your tongues move back and forth. Fast speaking is always my goal on the road to be good at English.
After all, sorry 'bout my bad English. And I really appreciate your help.