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As your mum's on a few forums herself she'll understand that there is the possibility of connecting with people who're not physically with you. She'll have bumped into people where she doesn't like them or she'll really get on with them and it was all down to font and text that she made those judgements. It's equally the same for you, you got to know someone through font and text, maybe even voice chat or actually seeing each other via Skype and from that, and through your online experiences together, you've both become close. This is something you can impress on your mum when you feel you're ready to talk to her. Another thing to bear in mind is that year almost 18, this means you're classed as an adult, this also means that if you do choose to meet up with your girlfriend it won't be up to your mum to stop you because, well, she's can't, you're over age and there's nothing she can do about it except express her dislike in the idea of you meeting someone from the internet.

If your mum already suspects you chat to people online, see how aware of this she is by actually asking her. Questions lead to a better understanding and from that, you'll understand just how much your mum is aware of what you're doing and this can help prompt the next set of questions. Dez made a really good suggestion of seeing if your mum could chat to your girlfriend and this could be the next set of questions you ask your mum, ask her if she'd like to talk to one of your 'friends' online and see how she gets along with them, let your mum get a 'feel' for your girlfriend and build up some trust. Putting myself in the shoes of a parent for a moment, I don't think I'd want my child to go meeting up with anyone, let alone someone from the internet, without actually having met them or knowing more about them first. Let your mum and your girlfriend get to know each other a bit first and see how things take off. If your mum gets to like your girlfriend you can explain to her that you're both in a long-distance relationship and see if it'd be ok for you to meet. If not, there's always the option of waiting until you're 18 and meeting regardless.