Why can you almost say a word but not quite reach it?

The word is right there, you can feel its shape, but it will not come out.

The tip-of-the-tongue state is a temporary retrieval failure, not a lost memory. You still hold the word's meaning and can often recall its first letter or syllable count above chance; only the route to its sound is briefly blocked. The word is still stored, which is why it usually pops up later on its own.

You bump into an old classmate at the market, picture their face and where they sat, but their name will not come. An hour later, paying for noodles, it suddenly surfaces.

Forgetting a word in the moment does not mean you lost it. The memory is still there, the access path is just jammed.

Instead of panicking that your memory is failing, relax and let the block clear, or chase the sound (first letter, rhyme) rather than the meaning you already have.

Tip of the tongue = the word is loaded, the trigger is stuck.

Learn the idea and practice English at the same time.