You should try to work out where the problem is coming from - the first syllable or the second. That is, is the interference coming from before or after the /t/?
If you can say 'settee, batty, putty', then the problem is coming from the /it/ part.
If you can say 'sitter, bitter,' but not 'sitting', then the problem is coming from the /ti:/ combination.
You need to define further what combinations you're having trouble with.
Is it just /iti:/ ?