Questions regarding the learning of speakers of spanish

I am a native speaker of mexican spanish.
I have studied the differences between this dialect and others, and the type I speak retains leismo, but not as strong as a spaniard.
My dialect did not retain voseo, mostly used in poetry.
My dialect had yeismo, but I am learning to distinguish "ll" from "y".
I pronounce c before i or e and z as s.
I am only a speaker through listening to my parents, as they both speak mexican spanish, but my mother pronounces y and ll as either english j or english y, rarely like the s in measure. My father only uses enlgish y.
When I was young i went to school in mexico, but since moving to the U.S., I tend to have problems with the verb "decir"
My question here is how hard would it be for me to learn another romance language, and which one should i start with?

Brazilian portugues... no doubt at all. It's the major latin lenguage closest to spanish.

Perhaps you already know it (just listening and reading, speak and write is harder), watch brazil internet tv, and read brazilian portugues on line to test it.

good luck...

Italian and Spanish may sound a bit more similar even thought they have significant differences phonetically but Spanish and Portuguese are much more similar grammatically, syntactically and the vocabulary is more or less the same with only phonetical differences and some false friends. They might be even considered two dialects!

Komtu, I would try to learn a little of bit of Latin, as this furthers consolidates your knowledge of English and Spanish. However, if you would like to learn Portuguese or Italian as the others suggested...go for it; however, if you learn better by aural & familiarity then I would suggest Italian but if you learn better by reading & analysis then go with Portuguese. By the way, your description on the Mexican variant of Spanish is quite true.