Myself is a reflexive pronoun that refers back to I in a sentence. If a sentence uses I, then myself can be used later in the sentence to refer back to I. If there is no I in the sentence you can’t use myself.
David, Danny and myself went to the seaside.
(myself isn’t referring back to I)
David, Danny and I went to the seaside.- I went to the event by myself.
(myself refers back to I) - I reward myself with a beer after a run in the park.
- I myself have never travelled outside the United States.
(myself immediately after I is best avoided but can be used for creative emphasis)
