var p12, err = base64.StdEncoding.DecodeString(`base64-encoded-pfx-file`)
if err != nil {
	panic(err)
}

blocks, err := ConvertToPEM(p12, "password")
if err != nil {
	panic(err)
}

pemData := []byte{}
for _, b := range blocks {
	pemData = append(pemData, pem.EncodeToMemory(b)...)
}

// then use PEM data for tls to construct tls certificate:

cert, err := tls.X509KeyPair(pemData, pemData)
if err != nil {
	panic(err)
}

config := tls.Config{
	Certificates: []tls.Certificate{cert},
}

// use tls config for http client