First of all, you should go see a Speech-Language Pathologist. If you feel that your difficulty with articulation is causing people to tease you, then that is a reason to intervene. I'm a student speech therapist, so I'll try to give you some help, but going to see a SLP would be best.
The way you are describing your 't' sound - 'crisp', 'blends into a 'd'' - makes it sound like you aren't aspirating. Sounds are said to be 'voiced' or 'voiceless', and sounds are classified by the way in which they are made. 't' belongs to a group of sounds called plosives. All the English plosive sounds are 'p', 'b', 't', 'd', 'k', and 'g'. Think of an explosion of air - hence plosive. Each of these sounds form a pair - 'p' and 'b'; 't' and 'd'; 'k' and 'g'. Each of the pair are pronounced at the same place - 'p' and 'b' with the lips; 't' and 'd' with the tongue just behind the teeth; 'k' and 'g' with the back of the tongue touching the top of the mouth at the back. What makes these sounds different is one is voiced, and one is voiceless. 'p', 't' and 'k' are voiceless; 'b', 'd', and 'g' are voiced. If you want to hear the difference between voiced and voiceless sounds clearly, touch your adam's apple lightly and say 'fffffffff', then, still touching, say 'vvvvvvvv'. You should feel vibration with the 'v' sound. This is voicing.
What relevance is this? Well, English voiceless plosives - 'p', 't', and 'k' - are pronounced with a puff of air. This is called 'aspiration' as I meantioned earlier. If you have no trouble saying 'p' and 'b', then try saying the word 'bin' followed by the word 'pin' with the palm of your hand held in front of your mouth. You should feel a puff of air hit your hand for 'pin' and not for 'bin'.
When you say that your 't' sound is crisp, it sounds like this puff of air is missing from your 't'. Without this puff of air, 't' can sound like 'd', especially in the middle of words like most of your troublesome words are.
I am presuming that you know where to place your tongue for the sound 't' and 'd'. If so, then just try to make the puff of air more exagerrated in the words you said you had troubke with, until it begins to sound like a 't'. When I say exagerrate the puff of air, I mean make it like the word has a 'hh' in it. So 'metal' would be 'methhal'. Not 'thh' as if it is 'thumb'. but the way you say 'h' on its own - add this 'h' to the 't' sound. I hope that makes sense!
As for 'th' sound, this sound is generally very difficult to make. The tongue tip should be between the teeth, but not too much, and not too little. Try to make the sound of a 'leaky tyre'. If your 'th' is sounding more like 's', then stick your tongue out a little but more between your teeth.
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