The hedonic treadmill is very real. If you are trying to get to happiness via achievement you will either fail to achieve the things you want, or you will realize after achieving them that they haven't made you happier. I've achieved things in my life. I didn't have any major failures until I was in my mid-20s. But no matter what achievements I had (or failures), the changes to my happiness were always temporary.
I pulled on that intellectual thread and learned that this is exactly what you would expect based on the research. Achieving your goals only achieves short-term happiness, and if you base your happiness on that, eventually you are going to reach a goal that stops your happiness dead.
You have to find the things that make you happy, and none of them have a long-term impact. They are the things you can do to maintain a consistent level of happiness, if you put in the consistent work. For me, the biggest impact on my happiness is regular meditation and regular exercise. Research also supports gratitude journaling and philanthropy. Simple. But NOTHING has a long-term impact.
To reiterate, the things that work are the things that give you a little boost of happiness every day, and if you find the right ones you can get to a level that is good for you through constant maintenance.
This doesn't mean achieving your goals is unimportant. Anyone who knows me knows that I have a dogged persistence when it comes to achieving goals. But I don't expect them to make me happy. The key is to recognize that your goals reflect your values, the things you believe are important whether or not you expect them to make you happy.
Purchase your happiness separate from your goals, it's much cheaper that way.