What should I do if my foreshadowing was too obvious and readers picked up on it immediately?

Foreshadowing is not used in a mystery to lead towards the final resolution of the mystery. Foreshadowing is used in novels to make unexpected events appear more plausible instead of random.

In a mystery, what you need to do is plant clues (or evidence) that point both at the final resolution (e.g. the murderer) and at the same time at one or more other possible outcomes (e.g. other suspects). You keep the clues ambivalent until the very end, where the final clue brings it all together and exludes all the other outcomes that, before that final clue, had been possible. You also plant false clues that actually aren't clues at all but only appear as if they were, while in reality they are there only accidentally and are not related to the mystery at all.

In short, you do not foreshadow the resolution, instead you distract and confuse the reader with both inconclusive as well as misleading clues.

Go a step further

It's good to reward readers for correctly predicting your plans, but you want to keep them on surprised and wanting to know what happens next.

So rather than stepping back from your planned reveal, step further. Find a way to fulfill what your readers predicted, but in a way they didn't expect.

For example, in the first Harry Potter book (spoiler alert), even if you correctly predicted that Quirrel was the villain of the story, you probably didn't expect him to have Voldemort attached to the back of his head. You can correctly put together the information you have and still be surprised by the info the author didn't give you access to.

Nothing. Keep calm and write on.

The level of trope savviness and theory-crafting skill of contemporary readers (many of which are also amateur writers) is the highest in human history. We all got sucked into the TvTropes binge, we all participated in internet forums, we all saw fiction stories dissected to their most base elements.

We were also exposed to the highest amount of content in human history. I read somewhere that 90% of all books ever written by humanity were penned in the last decade.

It all gets aggravated because people comment, talk, and exchange information about the fiction, usually in the very comment section (if it's an online novel).

Unless you are gearing your fiction to contain some big brain switcheroo and meta diversions to keep people off the tracks, someone will figure it out and then comment the hell out of it.

Hells, even guesses can get it somewhat right.

So, put the blame on the plastic cubes that work as a horrible interface between your brain and the digital paper-substitute. Castigate them thoroughly and keep on writing.

You mention you are a new writer. The only way to stop being a new writer is to write more. And don't worry much about the quality of your works (to the point of anxiety and loss of sleep).

In the voice of teenage Emma Watson: "Or worse, HIATUS."

I have a maxim:

In creative writing and software coding skill, the only real measure of progress is to review your old works. If you ask yourself "what manner of imbecile wrote this?" and the answer is "me.", then you know your skills have improved. Be your worst critic.

-- Mindwin, circa 2025

Mislead them. Read Our Mutual Friend (Charles Dickens). Dickens starts with a mystery. He drops clues here and there. Half way through he takes the reader into his confidence, and tells them what was really happening. At the end we realize that the story isn't what we thought, and there were other things going on that we didn't expect.