Avoid not sure as it implies inferiority. It says you have little to no confidence in your abilities.

Rephrase:

• Could you spare half an hour this afternoon?
• I’m not sure about the location. [uncertain without reason]
• We need to research more about the location. [expresses doubt and looks forward]

Avoid sorry and respect yourself and your contribution to any conversation or subject:

• I'm sorry but am I disturbing you? I would like to ask you a couple of questions. [weak]
• I would like to ask you a couple of questions about the Drax report. Would now be a good time?

There is a time and a place for apologising and requiring funds due to no-one’s fault is not one of them.

• I’m sorry but we need extra funds for database development. [weak]
• Due to unforeseen circumstances, we need extra funds for database development. Let me explain …

Be careful with stuff: it is vague; choose a more precise word:  

• These reports are not what we need. [strong]
• Sorry, I can’t make the meeting this afternoon as I have some important stuff to do. [vague]
• Sorry, I can’t make the meeting this afternoon as I have some important reviews to write. [strong]
  

Be careful with suppose: it can show indecision, indifference or that you do not care. If you are not sure, say so and what you can do to solve the issue:  

• This internet connect is unsecure so we shouldn’t send any sensitive data.   
  
• As we don't know if the internet connect is secure, we shouldn't send any sensitive data.
  
• Although there is much to do, everything will be ready for the meeting next week.
  

Be careful with should: it can show weakness, indecision or lack of commitment. In this context the speaker gives the impression of chaos and indecision.

If the meeting is important and your presence needed, commit and eliminate excuses:

• I have a meeting tomorrow.
  

If you need to soften the statement, prefix with I’m sorry:

• I’m sorry but I have a meeting tomorrow.