First things first: you need to talk to your manager. You need to ask about the direction the team will go in and the timeline for the future.
When I first took on this position you assured me that it would only be temporary. Is there a plan to hire/promote a new team lead? Can you give me an idea of how long I would remain in this position?
Change that wording as you'd like. The key point is to get a sense of your manager's goals for the position and the team. Now, most people in your position would ask if it was possible to make the temporary promotion permanent (and include salary negotiations etc.), but if you truly feel that you're not ready for the role or never want to move into a management position, you need to make that clear. That's because it's an easy assumption to make that you'd welcome this opportunity so you need to be explicit.
I didn't mind taking on this position temporarily and I've done my best to keep things running smoothly but...
- I feel that I'm just not ready to take on this kind of responsibility with my current experience
- I've realized that management is not something that I'd want to move into and I'd like to return to [whatever your previous role was/did]
Can you give me an idea of the plans you have for this position in the long-term and how much longer I'd be in this role?
Keep in mind that this is a career-limiting move! If you just feel uncomfortable with your new role then I highly suggest the trite-but-true "fake it till you make it" approach. Ask for more help from your manager and regular feedback. Read some management books or blogs and focus on developing your soft skills.
In about 95% of all industries and jobs, a career will involve managing others. Turning this down now will make it harder for you to get that kind of promotion in the future. If you're working for a company that has an "up or out" system then turning this down may cost you your job if you do it poorly. In that case you have to focus on "I want to do this but I don't feel ready and want more experience in X, Y or Z first".