am i an abuser/abusive person

The more you brood on this the more overwhelming it will become. You did something you regret. It's in the past so let it go. You are not an abuser because you were abusive once. Although it was nasty no bones were broken and there was no physical harm done.
Forgive yourself and get on with your life.

I am so sorry that you have let your regret over one mistake dog your life and doubt your own character for 17 Years. Please cut yourself some slack, forget about this and move on.
You are definitely NOT an abuser to spit at someone once 17 years ago. It would be different if this were continuing behaviour but clearly it isn't.
I imagine many people, when angry , have committed some small misdemeanour like this that they regret.
The proof of your pudding is in the fact that your partner stayed rather than left.
I don't know how old you are but did you ever see a child's programme also watched by adults called TISWAS? It featured a wonderful character called "Spit , the Dog" .Have a look at it and😊 laugh if you can
You mentioned mental health issues and I hope that if depression or any other medical condition is in any way responsible for this destructive thought pattern then you are able to get the help that you need.
Best of luck !

Perhaps you need to question why you are constantly thinking about this one event so long after it happened and how many other minor events are still swirling around your mind long after the event has been forgotten by everyone else

I think you should seek help to deal with that aspect of the problem. It seems to me that this is a mental health issue rather than an abuser problem.

You are NOT an abuser. Yes, you did something really gross (I've had it happen to me by a stranger and it really is very unpleasant) BUT you have never repeated it and I can't help thinking you've allowed this to take up head space for far too long. I've said and done some things I'm really not proud of and every now again they crop up in my brain and I quickly swat them away again.

Talk to your partner about how you feel and ultimately try to let it rest. It's taken up too much of your thinking time, clearly. You need to find peace with this and if the rest of your post is anything to go by, your BF might not even remember the event.

Be kind to yourself. You're still young and as I said to a friend recently "I would hate this to become the thing that rules the rest of your life. You have one life. Go and live it".

I hope you find some peace soon. Big hugs X