I think you went assassin too early. It will now be difficult to get good hitpoints & impossible to get many traits & spells without changing. Assassin is an excellent role but you need some training 1st.
I also have a level 15 assassin but he has been at least level 15 in all the other intermediate roles & 21 as a barbarian for the extra hp. He has about 370hp but anything over 300 would be good.
If you have a previous saved game it would be worth going back to it. You can go back for the dagger any time. If you haven't you can always aspire to be a zenmaster at the end of the game.
If you do decide to change roles again make sure you get all the quests that only an assassin can do.
__________________
|