It's interesting, in the church, how we react to masturbation. The older I get, the less worried I am about it in terms of being "serious". It's a normal (bell curve normal) part of growing up. We ALL investigate our bodies. So, friend. As far as I can tell, you are just about as normal as they come.
So lighten up.
The law of chastity is NOT about shaming sexual feelings and behaviors. And it's NOT about suppressing them either. It is about bridling them.
That means that our sexual feelings are obedient to us, instead of us becoming obedient to them.
We all were given a body when we came to earth. And for most of us, this human body is a sexual one. In addition, God gave us a sexual stewardship. In my experience, learning to master the sexual passions is a lot like riding a bike. You gotta get the feel of the balance, and then you can operate the vehicle appropriately. And that means getting OK with sexual feelings and sometimes sexual mistakes.
As I read your OP, I feel that you are stuck in a thought pattern about sexuality that is stifling and shame producing, and something that really won't serve you in your life. Perhaps a little marriage will help. Maybe not. Just know that masturbation isn't the end of the world. It just means you are a human with another regular everyday opportunity for growth.
I want to add one vote for telling your sweetheart. BUT, before you do, get ok with yourself and this minor mistake and clear about the non-serious nature of this mistake. Trust me! You are not the first priesthood holder to do this. And you won't be the last. Forgive yourself and open your mind to a less rigid view of this kind of sexual behavior. Understand how this failure is a really a fall forward towards building your bridling skills. Then, do go and discuss it with her. It's an awesome opportunity to discuss sexuality together and to build trust by sharing life experiences with each other. If your sweet heart gets freaked out, don't react by feeling shame or beating yourself up. Be patient and breath. Ask her questions about why she is reacting they way she is. See if the two of you can grow together as you talk.
AND THEN...like others have said...relax and forget about it.