How to improve mindfulness of thinking? Recently I've been facing problems with trying to remain mindful of any thoughts that pass through my mind. This problem is particularly prevalent in my formal meditation, where I only realise I am thinking half way through the thought, or even when it has stopped. I also cannot remember the first parts of the thought pattern, what made it to arise, etc. Any tips on keeping a more stable eye on this 'frame of reference'? Thanks.
My understanding is the 3rd frame of reference (cittanupassana) does not involve observing thoughts (vitakka). It involves knowing whether the mind-heart (citta) is defiled or not. Defilements (kilesa) are not thoughts. Although there can be 'defiled thoughts', 'defilements' (kilesa) are the energy/drive/mood of greed, hatred &amp; delusion, which are not thoughts.
For example, when there is sexual lust, anger or fear, the energy or vibrations that runs through &amp; simulates the physical body with lust, anger or fear, giving rise to all kinds of physical reactions &amp; tensions, is not 'thought'.
The teaching about the 3rd frame of reference does not mention thoughts. It states:

And how does a monk remain focused on the mind in &amp; of itself? There is the case where a monk, when the mind has lust, discerns that the
mind has lust. When the mind is without lust, he discerns that
the mind is without lust. When the mind has hatred, he discerns
that the mind has hatred. When the mind is without hatred, he
discerns that the mind is without hatred. When the mind has
delusion, he discerns that the mind has delusion. When the mind is
without delusion, he discerns that the mind is without delusion.
When the mind is constricted, he discerns that the mind is constricted. When the mind is scattered, he discerns that the mind is
scattered. When the mind is enlarged, he discerns that the mind is
enlarged. When the mind is not enlarged, he discerns that the mind is
not enlarged. When the mind is surpassed, he discerns that the mind is
surpassed. When the mind is unsurpassed, he discerns that the mind is
unsurpassed. When the mind is concentrated, he discerns that the mind
is concentrated. When the mind is not concentrated, he discerns that
the mind is not concentrated. When the mind is released, he discerns
that the mind is released. When the mind is not released, he discerns
that the mind is not released.
Satipatthana Sutta

To be able to observe clearly or meticulously the arising, existing &amp; passing of thoughts requires a high degree of concentration (mental collectedness/clarity/stability). The teachings state:

And what is the (mental) development using concentration (samādhi) that, when developed &amp; pursued, leads to mindfulness (sati) &amp;
clear-comprehension (sampajaññā)? There is the case where feelings
(vedanā) are known to the monk as they arise, known as they persist,
known as they subside. Perceptions (saññā) are known to him as they
arise, known as they persist, known as they subside. Thoughts
(vitakkā) are known to him as they arise, known as they persist,
known as they subside. This is the (mental) development using
concentration that, when developed &amp; pursued, leads to mindfulness &amp;
clear-comprehension.
Samadhi Sutta

A tip I can provide is to develop concentration (samadhi), such as by using Mindfulness With Breathing (Anapanasati).
An analogy is as follows. If you wish to scientifically examine water, ideally, you take a drop of water &amp; place it under a microscope. You do not walk into a raging surf (in the ocean) with a microscope. Similarly, to observe thoughts in a meticulous manner generally requires a large amount of concentration and a small amount of thoughts.