With 2025 fast approaching, many are pausing to take stock of 2024. For me, I’ll be looking at the moves I made in my Stocks and Shares ISA, judging which made me money (at least so far), and which didn’t.

Already though, I’m aware of a loss-making error that has taken the shine off my overall portfolio performance.

Here is the ISA mistake I’m keen to avoid repeating in 2025.

Doubling down

You don’t have to get it back the way that you lost it.

Warren Buffett

Axon Enterprise has been my biggest winner in 2024. Shares of the Taser maker are up a stunning 154%!

They’ve now surged 782% over five years.

However, I culled this top performer a while back, taking some profits as it mushroomed into a mammoth holding in my ISA.

Now, there’s nothing inherently wrong with that. It’s still a top position in my ISA (and SIPP), and I’m more than happy with it. It’s how I recycled some of those gains that’s come back to haunt me.

That’s because I added to my holding in mRNA vaccine pioneer Moderna (NASDAQ: MRNA) at the start of 2024. That position has fallen, along with my previous two purchases.

In fact, the stock has crashed 71% in six months and 45% over one year! The stock has turned into a giant red weed in my portfolio.

All in all, I’m down 65% on this share. Ouch.

What’s gone wrong?

Moderna was one of a small handful of companies whose vaccines came to the rescue during the pandemic. Its mRNA platform enabled the rapid creation of Spikevax in record-quick time.

It was the potential scalability of the biotech’s platform, and what diseases it could address outside of Covid (including cancer), that attracted me to the stock. But Moderna’s new vaccine for respiratory syncytial virus (RSV) hasn’t done very well, while Covid sales continue to fall off a cliff.

In August, the firm lowered its annual sales guidance by as much as $1bn, to $3.0bn-$3.5bn. And it doesn’t expect to break even on an operating cash cost basis till 2028, rather than 2026 as originally planned.

Meanwhile, Donald Trump’s nomination of vaccine sceptic Robert F. Kennedy Jr. for a potential political role in health isn’t exactly helping sentiment for the stock. So there are plenty of risks here.

On reflection though, my investment thesis remains just about intact. The company is still targeting 10 product approvals over the next three years, while reducing its cost structure.

Successful product launches aren’t guaranteed. But I’m encouraged that the probability of Moderna’s drug candidates progressing from phase one to phase three trials is roughly six times higher than the biopharma industry average. 

This highlights the exciting potential of the platform. However, the firm will have to start turning promise into performance through better execution for the stock to rebound and do well.

Foolish takeaway

Looking back, what I did was cut the flowers (Axon) to water the weeds (Moderna). That was a cardinal investing mistake, and one which I won’t be repeating again.

Looking ahead, I see plenty of opportunities, whether in growth stocks or high-yield income shares. With careful selection, 2025 could be a year of substantial gains for those ready to seize these opportunities.