How do I (18M) live in the present within my relationship with my partner (18M?)

I, 18M, am currently in an 'exclusive' stage with a guy, 18M right now. We have been on 2 dates so far, with another 3 planned. We have been exclusive for around a month now.

This is my 1st relationship, whilst this is like his 3rd. Being my 1st relationship, I'm really struggling with how to navigate my emotions and feelings.

I am mostly finding it difficult to live within the present. I'n struggling to appreciate how things are currently moving and I'm struggling to 'go with the flow' as I keep thinking about the future of our relationship. For example, I mainly keep thinking about how this relationship won't be a forever relationship (because we're both 17 and I just doubt we're gonna date until we die), and this is really ruining things for me. In my head, I'm like 'well if we're gonna break up one day, what's the point of doing all of this?' and I keep thinking about breaking stuff off just because I have a nihilistic view on the relationship and knowing that it's gonna fall apart one day in the future just makes me feel like this is all for nothing.

However, this is just a me problem. He's actually a really healthy and loving person and I do like him a LOT. He hasn't done anything wrong and he hasn't really done anything bad. Likewise with me; things are actually going really well.

Although I can recognise this ^, I find it difficult to apply that within living in the present, as I still think about our future way too much and it's really weighing on my emotions.

TL;DR - how do I live in the present within my relationship and stop thinking about 'what could go wrong' sometime in the future?

Please can someone advise me on how I can appreciate 'going with the flow' and taking things as they are, without being pessimistic and thinking about the future of the relationship too much.