Kill tasks will get you to level 20 easily enough. Talk to any NAMED guard and SAY task. Go kill what he tells you and go back to him. When you finish the tasks for your level (you can do a number of tasks equivalent to your level), you'll have a good idea where to hunt to finish off the level.
If you are having a hard time soloing, chances are you have spread yourself too thin on your spell line specialization. One spell line should be at least 2/3 your level, so if you are level 12, at least one line should be specialized to 8.
I cannot stress enough for any class that in order to be effective either solo or grouping, you must concentrate your efforts. If you are a fighter, you must concentrate on one type of weapon or damage type. If you are a caster, you must concentrate on one spell line. If you are a hybrid, you must decide which skills you want to excel in.
Pure casters are the hardest because you only get 1 spec point per level. Therefore, you have little room to diversify before level 40 (when you begin to get half-level spec points).
If you haven't yet, check out the various guides on theurgists available either here or at classesofcamelot.com.
If you've gimped your character by spreading your spec points around, it's definitely not too late to start over. I would recommend starting a new theurgist and do it right. You may not want to delete your present character right away, but after you see the difference between the two, I don't think you'll want to invest the time to try to catch him up.