Firpo was playing in the lower leagues of Spanish football before Betis came in for him.
"I always had it clear in my mind that I wanted to be a footballer," he told Radio MARCA.
"It's taken a lot [of time and effort] and now it's all happening so quickly.
"If you told me two years ago that I'd be playing in [LaLiga Santander] and in the [Spanish Under-21] national team, I wouldn't have believed it."
Firpo was born in the Dominican Republic but moved to Malaga at the age of six, which was where his football career began.
"When I was nine, Malaga called me but by father thought I was too young," he said.
"I kept that desire of becoming a professional, but when I was 18 and watching young lads making their debuts on TV, I lost [enthusiasm] a bit.
"I even told my girlfriend that if no academy signed me that year, I'd quit. I'd decided [to quit], but in the last league game in the youth team, we played against Betis and they signed me."