Out of curiosity, how many role plays have you joined, and what sort of RPs are they?

Free is mostly shotgun simple posting so if you want something with depth you'll need to move up to casual. Casual doesn't have to be several paragraphs, only a couple. It also doesn't have to be a fast rp and that could be what's causing you issues. The RPs you're joining may be too fast and you're burning yourself out too quickly. Much like eating a 5 course meal in a single, short sitting.