I Can't Seem to Finish??

I'm fifteen and I've been masturbating for a few months now. I've only ever stimulated my clitoris and I've been using an electric toothbrush for the majority of my time masturbating. I could never get very far with my fingers (and I would be sore afterwards) but once I started using the toothbrush it changed the game. The only problem is, though, that I can't finish. It's almost as if it's too pleasurable. I get really close, but I start shaking and I can't hold it in place, it's just too intense for me. What can I do?? I really just want to orgasm at least once in my life. Low key I'm not sure if I have before and I'm also wondering how you know?