Is there a word for someone who believes things will always work out in their favour?

A good choice would be optimist, meaning:

a person who is inclined to be hopeful and to expect good outcomes

However, you might be looking for something stronger than that, and Pollyanna is a good choice. I don't think there is a commonly used version with -ish at the end. But it is mainly used as an adjective. "He never quits, he is such a Pollyanna." "My Pollyanna wife never sees a bad motivation in anyone."

The dictionary definition for Pollyanna is:

a person characterized by irrepressible optimism and a tendency to find good in everything

Three things to note:

• As far as I know it is always used with a capital letter. Etymologically it is the name of a character in the novel "Pollyanna" who displays such characteristics.

• Even though it is a girl's name it is freely used to apply to males too.

• Not really mentioned in the dictionary definition, but in my experience it carries a certain color of cloying and annoyingly excessive optimism. However, that is more my opinion, and others might disagree.

I hope my answer makes your day a bit better, but that might just be the Pollyanna in me. ?

If you want a less common and more specific word than "optimist" you might like pangloss for which the Lexico definition is:

A person who is optimistic regardless of the circumstances.

and the origin of which is given as:

Late 18th century from the name of the tutor and philosopher in Voltaire's Candide (1759).

The fictional tutor's full name and title are Dr Peter Pangloss who, in the novel, teaches that "All is for the best in the best of all possible worlds" and he is usually said to be a satirical parody of the mathematician and philosopher Gottfried Wilhelm Leibnitz with whom Isaac Newton disputed the discovery of the important mathematical process known as The Calculus.

Newton and Leibnitz discovered The Calculus independently in the late 17C but it was not possible to prove its validity at the time. Because of this Newton kept it secret but used it extensively in his own work. Leibnitz, on the other hand published it without proof. The resulting controversy over who discovered it first drove a wedge between British and Continental mathematicians for many years even though the notation used these days is a mixture of Newton's and Leibniz's notations.

The adjective panglossian also exists and can be used to describe a person, a philosophy or a movement holding an over-opitimistic view of the world.