A key idea in Stoicism is the dichotomy of control: being vigilant about what is under our control and what is not. Accepting what is out of ones control and taking responsibility for what is.

It's commonly said that our thoughts fall under our control. So take responsibility for them and change them.

This is a useful first approximation, but it can be misleading.

We have many automatic thoughts and feelings.  Before you know it, you may experience powerful sensations or thoughts. A familiar smell may remind you of someone you've lost. Before you know it, you feel sad. A painting may instantly cause feelings of inspiration. An insult may cause you to feel angry.

In response to this, it may feel natural to judge ourselves for experiencing negative feelings or having negative thoughts. However, this would be mistaken. Sometimes they are out of our control. For they were immediate and automatic. We could not have intervened.

Seneca's essay On Anger focuses not on these automatic thoughts (what he calls impulses), but what happens next. We get carried away by emotion or passion when we react to our own impulses negatively. When we accept them as true without reason.

An emotion, then, does not consist in being moved by the appearances of things, but in surrendering to them and following up this casual impulse. For if anyone supposes that turning pale, bursting into tears, sexual arousal, deep sighs, flashing eyes, and anything of that sort are a sign of emotion and mental state, he is mistaken and does not understand that these are merely bodily impulses…. A man thinks himself injured, wants to be revenged, and then — being dissuaded for some reason — he quickly calms down again. I don’t call this anger, but a mental impulse yielding to reason.

He continues: Anger is that which overleaps reason and carries it away.

We can accept our impulses. Our negative thoughts.

It's what we do next what matters. Don't react; respond.