My greatest RP failure is not being an assertive GM.

In every RP I've either served as a Co-GM for or have been the sole GM of, I've had great turnouts and had a solid cast of characters, but some things would happen and I would just...not be assertive. I'd just let it fizzle out or succumb to the majority's opinion. I know this is my own fault and it has been something I've wanted to correct. But the thing with me is I've never known the line of "what is too assertive" and "not assertive enough". 

Another level of this is how I end up being bored with my own ideas. This could happen either through repeated drops draining my inspiration for a roleplay and not feeling confident in the idea anymore, or feeling so drained creatively that I close it because GMing is stressful.