Sales are about confidence in knowing that you know what you're pitching. When I talk to customers I know that my knowledge will provide them with much value and that they will continue listening to me. Depending on how you sell, be it over the phone, through chats, or in person, you have to start getting comfortable through that channel.
If you're selling in person you need to go to more networking events and don't be afraid to talk about yourself or business in front of strangers.
If you're selling over the phone you should spend more time talking to people. Make a few calls to customer service for a company you do business with. Take notes on how they answer the phone, what you liked vs what you didn't like, and how you felt after the call. Get in the habit of feeling good when you hang up the phone and carry that over to your sales pitch.
If you're selling through a type of online chat or forum, familiarize yourself with sales words. Start taking notes on which words were more effective than others when pitching. What initiated a response? What words got the person to reply almost immediately.
DO those things and become more confident in your product/ service, your process, your brand, and yourself - you won't have to worry about not being a good salesperson.