'Well, when I was young, I couldn't afford to go to college, so I decided to attend college while working. I've decided on this for quite a while now.'
Perhaps you are referring to two different decisions, the one you made when you were young, and the decision you made to attend online classes from the Credit Bank System. That is not clear. It appears to create a tense conflict (I decided vs. I've decided), and to say something obvious (that when you were younger was quite a while ago).
When you say you decided on 'this,' that most naturally refers to the preceding sentence, rather than what follows.