My character is so weak

[Belhi]

Most of your problem is probably your daggers. Your still using lvl 5 daggers and they are getting kind of old. If you have no luck with drops you can often find a vendor in the town in the zone relevant to your level that will sell basic weapons and armor for gil. If that doesn't work your you I suggest burning down enemies generally one at a time.

Using Throw Dagger that you should get from your level 15 rogue class quest to pull them away and finish them one at a time. There isn't much advantage to killing multiple enemies at once at that level.

On that, did you know about the class quest? You get new skills from doing them and I believe they offered weapons for some of the quests too. To do them simply go back to the place you first got your rogue class unlocked.