Kia ora Nick
Many business owners go through a similar thing, I remember once a friend of mine saying she could sell her butt off for another owner, but when it came to her own business she found it really tricky to self promote.
Without knowing what your business is, here are some thoughts:

Self promotion is necessary for growth of course, here are some points to consider

1. If you can afford to get someone even part time, even a student who is a wizz at marketing, you could give someone else the task of doing some self promotion for you, you could be in the background helping with content, images and other things, but they are doing the ‘doing’ bits on a regular basis to build your own brand as owner of the business
2. Or you could take some time do get some coaching from a business coach around understanding and owning your strengths
I have just co written a book and have an online course called unleash your superpowers
https://www.switchedonlearning.net/book

A couple of other things to consider before you you move forward –

1. Join a business networking group, like BNI or a chamber of commerce, where you are encouraged to network and talk about your business
2. Definitely put a picture of yourself on your website and social media pages, with a short bio, people love to see the face behind the business, get some help around this, you can take really good images on phones, but get someone else to take a bunch and pick the one that you feel represents you.
3. If you need help writing a bio, get someone to help you who knows you well, ask your customers what they think your strengths are, do some strength profiling there are some good ones out there that help you connect to those so you can then talk about them.

Self promotion is often seen as ‘bragging’ in NZ but as a business owner we have to know how to do this well for the right reasons

Some reasons why you need to self promote even a little:

Builds trust in the brand and you as the owner of the company and in turn the whole brand
Business referrals are way more likely when people can tell others to look you up
Some people, not all, but enough will only buy from a company they can see has a personal touch
Adds that personal touch and personal face

I hope these points and tips help you move forward with this, I have some free resources in the are of developing confident and communicating strengths over on my website

https://www.switchedonlearning.net/