From the Catechism, for an act to be a mortal sin, it requires the act itself is of grave matter, and the person committing to know that it is seriously wrong and that he has the freedom to commit it. Since I cannot know your motives and the degree of your inner freedom, I cannot say if what you described is a mortal sin. I can however tell you if an act is of grave matter.

In the Catechism, paragraph 2352, it clearly states that masturbation, or self sexual stimulation, is an intrinsically and gravely disordered action. Strictly speaking therefore, masturbation without ejaculation is considered masturbation and so is of grave matter.

Now that you know, that means ignorance is not a factor. Whether it is a mortal sin depends on how seriously addicted you are, namely, how free you are able to fight it.

Pastorally speaking, I often don’t find defining an act to be mortal sin very helpful. If something is likely to be mortal, it is best that I treat it as a mortal sin. Otherwise, I would trick myself in trying to get away with it as much as possible, meanwhile risking receiving Communion sacrilegiously and delaying repentance and going to confession. That kind of attitude always ends up piling up venial sins quickly and almost always ends up in mortal sins worse than before. Treating it as a serious sin on the outset, unless a person is dealing with obvious scrupulosity, helps to not going down that slippery slope and nip the temptation at the bud. If that means going to confession every week before Sunday mass, so be it. I rather fight this than prolonging my distance from the Lord. I rather throw myself at His grace than to trust my own thinking when I’m deep in sin.

On the other hand, here are some wisdom from the Church in fighting for chastity:

1. As much as possible, remove the source of temptation. Get rid of all physical or digital images and videos and membership etc. that give you access to pornography.
2. Put good strong filters on all your electronic devices. Have someone else manage the password for you. Don’t kid yourself to have self-control or that it is ok to look at something only R rated. The battle line must be drawn at zero adult material.
3. Take on a devotion. St. Francis de Sales taught that once purity is lost, the only means to fight it is fervent strong devotion. It will likely be a life long battle. I highly recommend daily Rosary and one of the several novenas or litanies to St. Joseph, especially the holy cloak of St. Joseph. You can pick one from your favorite saint too. There are some amazing healing stories through such devotion.
4. If nothing seems to work, consider signing up to a 12-step program, and a peer/mentor support group. They are available everywhere in North America now. Look it up on the web.
5. If there is a strong attachment to the internet, I would also strongly encourage people really struggling with it to not keep their devices with them in closed spaces, or consider downgrading their internet data package, or downgrading their smart devices to dumb ones. This is only good for cutting down or delaying the trigger, but sometimes that may gain enough time to retake control and renounce the temptation.
6. Lastly, if you fall, do not permit yourself to receive Communion until you’ve gone to confession, and do not skip Sunday mass even though you might have to skip receiving Communion. This is meant to be remedial, to own up the consequences of one’s action, and so allow grace to move one to resist sin more resolutely.