Optimistic and optimist

Solution 1:

We would say I am very optimistic when you might say Eu estou muito otimista, while we would say I am very much the optimist when you might say No fundo eu sou otimista.

That is, you would use the noun when you are expressing a permanent trait inherent to the person (hence ser in Portuguese), but you would use the adjective when you are expressing a passing condition (hence estar in Portuguese).

This may be over-simplifying a bit, but I’ve deliberately explained it using Portuguese equivalents because I hope this will make it easiest for you in particular to understand the distinction you are asking about.

Saying that you are an optimist means you are an optimistic person in general, but saying that you are optimistic about something means that right now you are looking at that thing as being likely.