There is a girl that I am in love with, and she is going to marry soon, not with me. We are friends, and I don't want to lose her; however, it hurts. Can stoicism help me with that? If it can, how?
- Some things are in our control and others not. Things in our control are opinion, pursuit, desire, aversion, and, in a word, whatever are our own actions. Things not in our control are body, property, reputation, command, and, in one word, whatever are not our own actions. (The Enchiridion)
You have to decide whether you will follow this distinction or not. It is the very core principle of stoicism and has some really undesirable consequences involving personal relationships with others. It's up to you to decide.
Since your question is not very specific, I can't give you specific answer. Stoicism can give you Happiness which is also the ultimate goal of stoic philosophy. The whole of stoic philosophy can be represented as a path to happiness.
I strongly recommend reading The Enchiridion by Epictetus at first. It is very very short and after reading it, you will be equipped with a good sense of what a stoicism is and what it means to be a stoic. It is completely contrary to misconceived popular belief of emotionless person.