So, to summarize:
1) Get a weapon that's at or just below your level.
2) Don't use the blunt goblin sword/scimitar. It's not a good sword until you fix it up. A brass gladius (level 16 sword) has much better stats than the blunt goblin scimitar (level 27 sword). For example, the brass gladius has an Accuracy rating of 104; the blunt goblin scimitar has an Accuracy rating of 63. And at level 19, you can get a brass dagger, which has an accuracy rating of 112.
3) Accuracy rings. If you're tanking, you'll want to use rings that give you HP or defense, but if that's not working, grab some accuracy rings. For example, try some brass rings.
4) Accuracy food. At your level, try grilled carp, braised papira, tree toad legs, or even grilled trout.
5) Keep your gear repaired above 50%.
6) Some mobs are just harder to hit. If you're having trouble with one mob, maybe try a different one for a little while.
7) Raise your physical level. Seriously, I can't emphasize this enough. When your physical level starts increasing, you can max out your stats for your job level, and the mobs become so much easier. I think many of us that are higher levels now forget how tough it was to fight even yellow mobs when we first started out. So do some crafting/gathering, or just work on some other jobs, to increase your physical level.
8) Many jobs have abilities that increase Accuracy. If you take your Lancer to 20, you can get Feint. And Archers have Hawk's Eye, which increases your accuracy for one swing.
9) I don't think anyone else has asked this, but you are equipping a shield and using the Guard ability, right? When your shield arm drops, you need to press Guard again to raise it. (Sorry if you already know this. It just seems like you should be able to last a long, long time against green mobs, if you're using a rank appropriate shield and using Guard ability correctly.)
10) Try not to get too frustrated. Keep in mind that it's all just dice rolls going on behind the scenes. Sometimes you're going to get really lucky and hit every time. Sometimes you're going to get really unlucky and miss every time. One time, I was in a party with my Marauder, and didn't hit a single mob throughout an entire leve. Sometimes that stuff just happens.