How to loosen peanut butter in a sauce
If you add all the liquids and a big solid lump of peanut butter, you'll have a tough time getting it all smooth, as stirring the (thin) liquid parts won't affect the (solid) peanut butter lumps, and the lumps (once they're small enough) will just swim around your spoon and not break down further.

You need to gradually dilute the peanut butter with the other hot liquids, stirring each time until the mixture is smooth. That way at each stage the liquids won't be vastly thinner than the peanut butter, so stirring will properly mix the two. Once the peanut butter mixture is thinned to the consistency of, say, mayonnaise, you'll be able to add the rest of the liquids.

It's the same principle when making gravy from flour, fat and broth. If you add all the broth at once to the roux, you'll end up with lumpy gravy. But, if you add a bit of broth at a time, stirring until smooth each time, you'll never have lumps.

To fix your problem in hindsight, try running it all through a coarse sieve, or use a potato masher on it.