I don’t think this is a question on how to get aroused more often but why you might be feeling that way - may that be due to mental health problems (such as depression), lack of sex drive - which is perfectly normal - due to sexuality (being asexual), or even simply due to the fact you’re just not ready for sex yet.
But most importantly, your boyfriend (and future partners) should be more supportive with the fact that you aren’t comfortable to have sex. A relationship is built up of a variety of building blocks but the foundation should not be sex. With a foundation of a relationship being supported by sex only leads to a relationship where there is a lack of emotional support (such as being frustrated with the fact that you don’t want to have sex and yet not bothering to understand why that might be the case)