One of the biggest reasons prospective clients don’t proceed with a project after having you provide a thorough proposal is because they are concerned about their options if you mess up. Take that risk away: Offer a strong guarantee. Here’s how I do it in my consulting practice.
Something like this will do:
The quality of my work is guaranteed. If you do not believe I have met the mutually established objectives for this arrangement, I will continue to work toward those goals with you for no additional fee. If, after such an additional attempt, you still believe I have not met your objectives, I will refund your fees in total.
I borrowed this idea from Alan Weiss years ago, include it as boilerplate in every proposal letter, and haven’t looked back.
Is the above scary? Are you taking on more of the risk? Yes. That’s why it’s powerful.
Do you have to use exactly this language if it makes you too uncomfortable? No. But make sure you pick something that makes you a little uncomfortable. Otherwise it’s meaningless and not compelling. And therefore not of value.
(Also, you’ll have to adjust the language if your projects aren’t fixed fee — like mine predominantly are — but the spirit can be similar.)
Not incidentally, offering a compelling guarantee is also justification for charging higher fees. Accepting risk — taking it away for the client — has value. Clients aren’t simply paying you for your advice or your labor, but everything you bring to the table.
If accepting this level of responsibility for your work isn’t acceptable to you, that’s fine. Just expect to close fewer, smaller, and less profitable deals. Which is fine if it helps you sleep better.
But I sleep better doing things this way.
 You don’t have to go crazy. Try it on a proposal or two and see what happens.