Lately, I am making decisions based on imagining whether my future self is more likely to regret doing something or more likely to regret not doing it.
If your phone already works fine, are you more likely to regret buying an iPhone X or more likely to regret not buying one? Imagining spending over $1000 and then realising it only does the same tasks as your existing phone once you get it. It is easy to imagine that if you knew nothing of the iPhone X, you would not regret not having one.
If your phone is old and tired, suppose that the next time the battery runs out on you at a critical moment, it is easier to imagine regretting not buying the new phone earlier.
Unless it's broke, don't spend money fixing it.
I have a couple of times in my life got buyers remorse from a new phone or a new car when I replaced them and the old one was fine.
With "stuff" it is relatively easy - a rough rule of thumb is that you would regret having it more than not.
This type of decision making broadly aligns with what is already known about happiness - better to spend money on experiences rather than things.
If you are invited somewhere, even if you are not sure you are going to like it, better to go and regret doing it than to regret not doing it - it could be the best thing you ever did, after all?
Would you regret going out in your 20s versus staying home and not meeting anyone? Pretty easy to imagine you'll regret something you haven't done more than something you have done in this example.
What about if you regret spending your 20s working to build a business and never going out but then you have all the money you ever need and can retire at 30? More difficult to imagine - but is this the real decision you are making?
You have time. Do both.