Damn, cancer at 18, that's metal. I can't imagine what that's like, the closest thing is a couple people I know who went through breast cancer. I hope you have good support outside of imgflip...if not, we're here in whatever limited capacity we can be.
As for staying happy...uh...I don't even know how to BE happy, much less stay that way. It's important to let yourself feel what you feel, so it's good not to try to suppress things. It's also good not to get overly attached to feelings. Things are constantly changing so it can be good to embrace the change. Sit with your feelings--acknowledge them, don't fight them, but don't fight to hold onto them either. They're going to change either way.
Lastly, you're stronger than you imagine. No one knows what they're capable of until they're forced to deal with a situation. Just be ready to embrace whatever challenges life dumps in your lap and trust that you'll get through them, even if you have no idea how. And ffs please ask for help when you need it. Don't ever make the mistake of believing you're in this by yourself.
