By saying no, you leave yourself open to say yes to something even better, but that doesn't mean it's easy.
If someone comes to you as an investor, they have probably put a lot of effort into whatever they're wanting investment for, and you can tell.
They chose to be vulnerable with you when they decided to ask for an investment.
Your heart probably recognizes this, and wishes to reward their effort, even if your mind knows it's not a good idea for you to invest.
Bless you!