Is this above paragraph similar to your own inner voice? I hope so, but probably your inner voice isn't quite as peppy and may express doubts. It's okay and natural to have doubts, but problems arise when those doubts get in the way of fundraising success. If they are influencing you to set lower goals, not approach a potential donor, or forgo a grant opportunity then it's time to examine those thoughts.
It may sound cliché, but the old adage "If you don't believe in yourself, who will?" makes a lot of sense, especially when it comes to fundraising. You have to show people how worthwhile your organization is and why they want to be part of your movement, and if you have doubts tugging at the back of your mind they compromise your ability to be a confident presenter, conversationalist, writer, etc. Think of those infomercials you see where part way through you start thinking "Maybe I do need that." (The ones that always got me were for the Ronco Rotisserie--and I don't even eat meat!) I'm not suggesting you imitate their style exactly, but there is something to be learned from their confidence and ability to exude belief in the product. Your nonprofit is infinitely better than any product advertised on TV, so if they can be that confident surely you can, too.
Many times people get particularly anxious when approaching a donor for a large gift, because it feels very personal: it might be one-on-one, in a private location--perhaps even their home--and you're going to have to mention money, which can be a taboo subject. But if you think of your ask as an opportunity for the donor to fulfill their personal giving goals and support a cause they believe in then it isn't as scary. And it never hurts to remember another old adage: What's the worst that could happen? They could say "no." Of course the corollary is that they could say "yes," and isn't that worth the risk?
Fundraising & the Power of Confidence
How to Nail the Soft Skills of Fundraising: The Face-to-Face Ask