Sentir takes what you feel: a thing, or a fact you are sorry about. Sentirse takes how you feel: an adjective or an adverb that describes your own state.
Where the choice changes the meaning
| Context | sentir | sentirse |
|---|---|---|
| What follows it | A noun: siento frío. | An adjective or an adverb: me siento mal. |
| How you are | Not used for a state. | ¿Cómo te sientes? — the way to ask. |
| Apologies | Lo siento — I am sorry. | Not used. |
| Regret about a fact | Siento que no puedas venir — takes the subjunctive. | Not used. |
| Perception | Siento que algo va mal — you sense it. | Not used. |
| Emotions | Siente miedo — names the feeling. | Se siente asustado — describes the person. |