In some single words such as aesthetic the problem is the same and you will have to work on it Leeearn. But luckily there are some single words in which the th sound disappears, for example asthma and isthmus, pronounced asma and ismus. Can anybody think of more of these?
Great work. They are tricky sounds - you're right. The first sentence was clear, however I lost the second sentence.
It's tricky because the 's' is voiced and the 'th' is unvoiced.
When practicing, one thing you could try is slow it right down and feel how the 's' sound is voiced and that you need to change to the unvoiced 'th.' Really overemphasize the 's', then really over emphasize the 'th' and play around with feeling the difference.
Then bring it back to normal. It's the 'th' sound you really want to work.