First, we have a thread conveniently located just a few lines further down the screen in this forum titled "
[GUIDE] How to improve in osu!mania" which would have been the appropriate place to ask this question.
Second, CptBlackBird is entirely correct that the only advice anyone can give you that we know for sure will help people improve is to play more.
Third, I'm not sure which o2jam converts you played. O2jam has a wide range in quality from quite bad to very good, but it's entirely possible you only played bad maps. Alternately, it's possible that you decided that those maps were bad either because of patterns you don't enjoy, or because of issues with converting them from o2jam (which usually stem from the use of wanwan's kinda bad o2jam converter). Unless you've played more than ~1500 o2jam conversions you shouldn't generalize them as all being bad. (Both DJKero and myself have collections of roughly that size of o2jam converts)
Fourth, when searching for unranked songs, it might be worthwhile using the 'recommended difficulty' option on the beatmap search here:
It figures out roughly what difficulty you play based on your ranked scores, and lists maps with similar star ratings.
Also, pay attention to individual mappers, and periodically check to see if they have uploaded something you haven't played yet. I have a whole list of mappers who I will download anything they upload because I like their maps. While this doesnt help find maps by people who only upload 1 or 2, is's still helpful.
Normally I would suggest going out of your comfort zone and playing much harder maps than you're used to, but it seems you aren't afraid of getting lower scores (based on how few Ss and how many Bs I see in your top scores and recent accomplishments). If you still avoid them, playing songs you get Cs on m could potentially help some as well.
Apart from that, playing more really is the only way to improve. Unfortunately, getting better at this game isn't going to unlock some massive trove of difficult maps to play. Unless you want to play o2jam converts or BMS converts anyway.