To make it easy for people to help you please modify your post and use the code button </> so your code looks like this and is easy to copy to a text editor. See How to use the Forum
Also use the Arduino AutoFormat tool to indent your code for easier reading.
I don't know what you are sending to the Arduino but whatever it is this is a crap way of receiving it
if(Serial.available() > 0) {
int value = Serial.parseInt();
char stepState = Serial.read();
Have a look at the examples in Serial Input Basics - simple reliable ways to receive data. There is also a parse example to illustrate how to extract numbers from the received text.
The technique in the 3rd example will be the most reliable.
Also, you should separate the business of receiving data from the business of controlling your motor. It will make testing and debugging much easier. Have a look at Planning and Implementing a Program
I don't know Processing - I use Python.
...R