<human>: Give me the gist of the text in a nutshell

So I've been going out with this girl for about a month now, from the start she's been terrified that I'd leave her. She apologises all the time for talking too much, and has even told me that she waits a few minute before replying to my texts so she doesn't look obsessed... 

Since we've been going out quite a few girls have started to like me, and she's noticed it and keeps getting really upset. I've done my best to make her believe that I love her and nobody else, but it seems like everytime I manage to persuade her, another girl makes an advance. 

I was talking to a girl before(15/f), and she out of the blue sent me a nude. I told her that what she's doing was wrong and that I wasn't going to be a dick and send anything back. 

I want to tell my girlfriend about this because I feel guilty, she's told me before to tell her if any girls say anything, or ask me out, but everytime I tell her, I can tell she gets hurt.. 

If she finds out from the girl who sent it, she'd be devastated. But if I tell her, she'll be really worried. I have no idea what to do, I don't even know if any of this makes sense. 

So first of all, do I tell her about this or not? I'm pretty sure I should just forget any of this ever happened and move on. 
And secondly, how do I handle her being so insecure and scared I'll leave her.
<bot>: I'm with an insecure girlfriend, random friend of mine sent me a nude, don't know if I should tell girlfriend or not.