You call it nihilism because it seems meaningless for you, but I want you to notice how it can only be meaningless from a human survival perspective; a perspective which only sees meaning in the perpetuation of its own agenda. What is really meaningless about an infinitely complex dance of trillions of molecular machines, interacting with eachother and rearranging themselves and their environment in an ineffable display of creativity?
When I say "just a bunch of cells", I don't mean to be reductionistic or to subtract meaning from the equation. What I'm really saying is "but what about the cells?". The mistake would be to look at the phenomena of "animal" from a gross surface level view and forget the immense density of stuff happening at the micro and meta-macro levels. The mistake would be to not see the vast web of relationships in reality and how each part is dependent on the other. There is an absolute necessity in everything being just as it is, right now and forever. It's a perfect design.

You can also look at this from the perspective of neuropharmacology. The mind-body system generally downregulates the effect of exogenous stimuli (things coming from the outside) in order to maintain homeostasis (balance between internal and external).
For example, taking a drug that releases excess dopamine (say amphetamine) downregulates the body's own production of dopamine in order to reduce the excess dopamine signalling, leading to what is called "dependence". When you stop taking the drug and the duration is over, you'll have less dopaminergic activity in the areas that were previously experiencing excessive stimulation (come-down, withdrawal).
If we assume that dopamine plays a role in happiness, then you haven't actually "received" happiness from the outside by taking the drug. You've only traded short-term increase in dopamine for a long-term decrease, and you've also traded away your own ability to regulate dopamine signalling. You've only made your happiness more dependent on fleeting conditions (which are also progressively fleeting, because over time you need to take more of the same drug for the same effect, approaching infinity).
More importantly, your ability to be flexible in your functioning; to be alert and activitated in one moment, and calm and relaxed in another, diminishes. There is a selective numbing effect, because the drug only borrows the body's own mechanisms and resources in order to have an effect. You don't gain anything in the long run; you only externalize your own internal control and decrease dynamic responsivity. The changes are not self-consistent, and there is no potential for growth on its own, only degeneration.
To maintain a steady source of happiness, you want to "upregulate the regulatory capacity of the system". Instead of introducing more stimuli from exogenous sources, you increase your endogenous ability to respond to those stimuli. You work with the ebbs and flows of homeostasis by promoting resilience and stability. It's a long-term growth mindset and is the essence of spirituality and holistic health.
You start by looking at the base of the system and work bottom-up, being aware of natural cycles, systematically eliminating dysfunctional behaviors and promoting growth (get enough sleep, physical exercise, remove dietary poisons, eliminate mental trauma, train mental alertness etc.).
From this point of view, unhappiness simply stems from your own inability to regulate your own system. That is essentially the same as saying that happiness comes from within.

I see that many here think they are enlightened already and share what worked for them. I like that, but maybe none of them are enlightened, or a few are, or all are. And sometimes Leo chime in and say something like "you guys haven't even seen God's tits yet, keep going", and they will be like "but Leo, I'm nothing! There is no one here to keep going", and then they just agree to disagree