'Think of' generally is used in the context of thinking of an example that meets some criterion. e.g. 'Can you think of a word that has each of the vowels in alphabetical order'. 'Think about' means to consider something. 'Would like like to come to New Zealand with me? I'll have to think about that'. 'Think over' is similar to 'think about'. It may have a suggestion of a longer time of thinking. "can't help doing something" and "do without something" are very different. "can't help doing something" refers to an inability or unwillingness to stop doing something. e.g. 'I can't help drinking', or 'I can't help picking my nose'. "do without something" means being unable or unwilling to function without some item or being. 'I can't do without my phone'. 'I can't do without my father'.