In fleet, the tongue does touch the roof of the mouth and release, creating the /t/ sound, and then relaxes, allowing the air exhaled during speech to pass uninhibited.
On the other hand, when pronouncing /ts/, after pronouncing the /t/ the tongue remains close to the roof of the mouth with the tip of the tongue close to the front teeth as the air that is exhaled during speech is somewhat restricted, creating the /s/ sound that will help you distinguish between /t/ and /ts/.
Try pronouncing them to your self both ways to see if you can hear the difference. If this is successful, begin attempting to hear it in native speakers. If you have access to one, ask him or her to speak the different sounds, emphasizing the /t/ and /ts/ sounds.
/i:/ is the vowel that we find in the word FLEECE. I put that word in capitals because that is how that vowel is often referred to by linguists: the fleece vowel - or FLEECE for short. (This is not random, the word was specifically chosen for a number of specific reasons.) It is the vowel sound at the end of the word guarantee. In transcriptions of British English it has a colon [ : ] in the symbol to describe the length.
/ɪ/ is the vowel in the word KIT. It is known as the kit vowel - or KIT for short. It is the vowel we find in prefixes and suffixes, the bits we stick onto the beginnings and ends of words. So, for example it is the vowel we hear in --ing verb endings.
The vowel represented by /i/ at the ends of words in dictionaries is usually referred to as the happy vowel - HAPPY. This vowel may sound like either FLEECE or KIT, but is always short in duration.
If you say the < y > sound that we find in the word yes, and then say the < e> we find in the word end, the kit vowel is somewhere between the two sounds. This is the first vowel in the word infinitely. This word would sound very odd to a native speaker if it was said with a fleece vowel, /i:/! It would sound like a made-up word: eenfinitely.
The Original Poster asks if there is a big difference between these vowels. If we are talking about the physical difference between the sounds, the answer is: no. In fact, it is very unusual to have two vowels that are so similar in one language. They are very close together. In most languages these would count as one vowel. However, if we are talking about the meaning, or the effect on a listener, the answer is: yes! There is a big difference. There are very, very, very many words that we can be confused about if you say the wrong vowel. For example, the words peace and piss. Nobody wants to say Piss man!, when they mean Peace man!.
If you want to type IPA script, this website is very useful
Hope this helps,
Peace!
Best Answer
I pronounce the two words differently. I happen to be a native speaker of American English whose father studied French extensively and whose mother taught French -- but I do not know French myself.
I pronounce "confident" as "CONfidint", much like jimbobmcgee's transcription.
I pronounce the last syllable of "confidant" the way I pronounce the first syllable of "Ontario", or the way some people pronounce "aunt". In other words, it rhymes with "want".