Sounds like a spot of impostor syndrome, it’s very common and nothing at all to be afraid of – I’m no psychologist but there are plenty of resources out there to help you overcome the mental block.
Finding new clients is always the hardest part, do you have a portfolio of previous client case studies you can share on LinkedIn, or better yet, a website ranking well enough for local SEO consultancy / agency key terms?
In terms of practical advice, start small and build relationships with clients. A couple of hours a month can very quickly snowball to a day a week or more. Demonstrate your value and start to add in lines like, “I can also help with…” or “If we did this it would be more effective…”.
The best advice I would give is to maintain regular calls or emails with the client, outlining what you did the previous period, intend to do in the upcoming period and the expected results – Especially as SEO tends to be a slower burn than PPC. An additional email or call to talk about results. I found this way is great for keeping the client informed on what you’re doing, it also gives them a chance to feedback any questions or concerns. The results email I would always encourage the client to share with their wider team – Hence the separate comms, Sales don’t care how many positions you’ve improved, they only want to know what leads / sales it’s generated.
As always with this sort of thing, I’m of the belief they have brought you in to do something they cannot – There’s absolutely nothing wrong with increasing (or decreasing) your fees as long as you’re talking to the client regularly and able to justify changes in the project or type of SEO you are doing.