This is a difficult struggle, but let me assure you, you can overcome it (with divine assistance and especially the Blessed Mother). You will need to gear up for the fight, change the way you think and become militant about achieving victory.
Right now the temptation is to find a rationale that will make giving in to the temptation easier. Don't do it. The truth is you are going to fail here or there, but this is a war, with many battles, some losses, some victories, but you need to start to create more and more victories.
Your body is used to producing a certain amount of sperm and ejaculate on a regular basis, so every couple of weeks or so your libido rises and you feel the impulse very strongly. If you act out on it, the pattern will not be broken. The more you resist, the greater effect you will have on changing the way your body deals with the hormonal and libidinal rise. Eventually things will calm down, and when there is a build up, your body will take care of it naturally, by re-absorption, some ejaculate during or after urination (and even on occasion during wet dreams).
What you are aiming for is the virtue of purity. Virtue is won by many small battles; hundreds of temptations beaten back into submission. It is a holy habit.
Vice is the opposite: indulgence at a great frequency. Even secular motivational speakers say that creating a new habit takes at least (7) seven weeks to start to hold.
Your use of the Rosary is great, but it is not enough. You need to do extra. Go to Mass whenever you can during the week (at least once a week), not just one on Sunday. Offer Mass up for the intention of purity. Religiously avoid anything that is a trigger, especially porn, soft porn, or anything else. You will find that your stress will go down when you spend time just meditating and relaxing in church in front of the Blessed Sacrament because as you give God control, you will realize it is not all up to you. You may be telling yourself, you are too busy for all this, but do it anyway and your life will change.
As far as the mortal nature of the sin, your culpability is known only by God, but suffice it to say that sin done of a serious nature, with full consent of the will, is generally a mortal sin. The fact that the temptation is super strong doesn't reduce culpability, it just means you need more grace — for which Jesus is always ready to give.
I have heard from another friend who told me when they were young, single, and struggled to be pure, when they were living in their apartment, they could hear the neighbors above them having sex directly above them. They said it was like being injected into a porn studio without their consent, and their body wanted to participate. They empathized with St. Paul, the spirit was willing but the flesh was weak. (Romans 7:8ff) They had no choice but to pray in that moment for God to intervene and help to keep them pure. After they focused, their body would slowly calm down. It wasn't easy, but they resisted. — (How?) — Only by the grace of God. He will be there in those very tempting moments but you have to invite Him in. Too often we are more like Adam and Eve, and want to just run away and hide in our nakedness and shame.
- If, while they were at the tree talking to the serpent, they asked God for some help, do you think He would have abandoned them?
We would probably be living in a different world had they made a better choice.
My last bit of advice: Get an accountability buddy, or friend to share your struggles with. Carrying it alone makes it even more difficult. You also should try to fill your mind with other things like growing in knowledge of Scripture. Read the Bible; a little every day. All these things will lead to victory but it is a process and you have to know that like any good general, you must assess the battle plan every once and a while and evaluate how it is working.
If you are losing battles, adjust the plan, bring in more armor. Never surrender!