A language that you can understand, practically without previous
studies, can be hard to learn actively. If you have another alternative
you may well never get active in language A. This is my situation with
Norwegian. I understand practically everything I read or hear in it, but
that doesn't mean I can speak or at all write it.
Interlingua is to some extant of the same sort. If you know French or
Spanish you can be well tempted to use that language instead of putting
Interlingua to active use.
So you feel that you have to work more with it, but compared to a
language where you have to learn everything, it is quite easy.
Unfortunately there are few teaching aides in interlingua. Interlingua a
prime vista and Interlingua grammar are good, but the best book is to my
mind Ingvar Stenström's Interlingua, instrumento moderne de
communication international. It is intirely in Interlingua but there are
commentary parts in a lot of languages, even English, as far as I know.
Perhaps your temperament is for Novial. Then you can use that. I will
understand if I were the target of your information. Using it actively
is my problem!