tips on getting over gf cheating on me with a friend of mine who i once considered my little brother? ?

Make sure they're both out of your life. That's all you can do. If you want, you can take it a step further and be in a relationship without getting your emotions involved, but I don't recommend it. So your choice is either be forever alone, or get over it and realize that not everything is in your control and that people will always have the ability to hurt you regardless of who they are.

Same thing happened to me and I'm sure millions of others. Just tell yourself they were assholes and move on with your life. That's what I did and I never gave them another thought until now. There are no guarantees in relationships and marriages. Some last months, decades or a lifetime. Just try and make it work, that's all you can do and if it fails, then accept it and find happiness elsewhere.