I've been struggling with this for a while and i finally decided to join GYE and work on fixing myself. My current struggle is that randomly-without any provocation, i will get hirhurim in my head which will cause me to be mz"l. Even if i try to push on and forget about it, they keep nagging at me. B"H this issue is not internet related and it is not all the time (there are months where there are no hirhurim). Does anyone have any tips/advise on 1. how to prevent these hirhurim, 2. what to do once these hirhurim pop into my head?

It is impossible for a person to think two different thoughts at the same time. Therefore a person can easily banish impure thoughts without actively confronting them. That is, he can elect not to think the wrong thought, rather to think about something else concerning Torah or service of Hashem, or even business or some other subject. In this way he will automatically ("m'meila") free himself from any negative thought that comes to him.

 A person should not become discouraged when he sees that impure thoughts are afflicting him, even if they are awful and embarrassing. To the contrary, this exactly is where the key to his tshuva and reparation lies. Specifically through this test, that impure thoughts come to him and he vanquishes them - specifically here is the key to his salvation.

I second DavidT's words and go even further.
The Alter Rebbe in the Tanya says a person should not despair by having impure thoughts, on the contrary, he should rejoice! G-d gave this person a special task. One that even tzadikim can't accomplish (they don't have impure thoughts), and that is to divert the thought and thus subdue the klipot. That causes G-d an immense pleasure. He should also know that the impure thoughts come from his animal soul, not from his G-dly soul. So it's only natural that this person and 99% of the world population should have impure thoughts. Even gedolim have admitted they have hirhurim. The key is to realize you can banish them, and that's an enormous mitzva. When it comes, don't engage in it. Don't try to understand it. Just try to think about anything else, and occupy yourself with an activity. As a further geder, try not to be so alone. Take notes when and how do you fall. If you can avoid being alone, all the better.