Hi Aidan; welcome to TeenHelp! Thank you for posting about this. I hope what I have to say can be of some help.
Honestly I'd say you're looking at it the wrong way. Of course we want our relationships to last for a long time, especially when we care for someone as deeply as you seem to care about your girlfriend. But focusing on the anxiety of potentially losing our partners and what we can do to keep that from happening is counterintuitive. I've found, from personal experience, that the best thing to do in relationships is to be present. Take it day by day, moment to moment. When you are with your partner, be truly present with her, and enjoy the time you're having in the moments you interact with her. By focusing on the present you truly get the most out of that relationship. Good times and challenges will come, and those challenges will be frustrating and perhaps cause anxiety, but if you're present with her now, and going forward, you will build the foundation to weather those challenges. If you're worrying about the longevity of your relationship you're going to be so freaked out it might end that you won't be able to effectively communicate with her during those times, and it will make it more difficult to get through those times.
I know this probably isn't what you want to hear. The fact of the matter is (and I don't mean it patronizingly) that you are both very young. In all likelihood it won't last, not because of a failure on your part, but simply because people change so much as they grow. This is especially true regarding the change that happens between the young teen years and who you eventually become as an adult. I am not saying it's impossible, or that it automatically means y'all won't make it because you're young. People can, and do, stay together from the time they are very young. I am just saying make the most of what you have, in the present. I think you'll be happier that way, and it will take pressure off both of you. Continue to help each other and communicate well and it's much more likely to be a longer lasting relationship.
Please feel free to
PM me if you have any questions or concerns. I am happy to help as best I can. Take care!