What does it mean when an ex said you deserve someone better/I'm not good enough

It means they are not willing to put the effort in to keep the relationship going. It's a mixture of the relationship not being as good as they want, and there being other people that they are attracted to. Don't worry about it. Over is over.

If they've already said it, does it really matter at this point?

First breaku[p, being let down gently will keep my self-esteem much more intact than knowing the truth was I was an inferior human being or something in their eyes :P

So, 'you're not good enough for me'?

Well in that case you feel grateful that your first break-up was so kind! At least they wanted to spare your feelings. I would infer from this that you did nothing wrong but she just wasn't in it - nothing you did.

If you think about it like that, it's more like 'you're not good enough for me, only a relationship with the constant excitement of a one night stand would be good enough for me' and I don't think relationships like those exist.

They just don't want what you want, and they kind of blame themselves for not wanting a relationship. They mean that you would be great relationship material with the right person, but you're not what they want right now.

They are kind of saying that you're not good enough, but they know that's not the truth. They probably would have done the same no matter who their partner was. (unless their partner is unattainable, which defeats the point of them being a partner).

She sends me mixed signals though.
She texted me the night after saying I love you and that's why I'm letting you go
She told me that she was finding it horrible at home and just wanted to cry because she didn't know who she was and where she belonged
She said i'I'm nothing without you' and 'I'm scared of losing yu'

Then when we actually met up she barely spoke to me, saying she was 'stressed' with uni work and stuff, apparently I annoyed her for getting the time wrong (in fact she texted me a new time, which I didn't receive because my phone ran out of battery; I arrived for the old one)
But she got there half an hour late and came up laughing and joking with her guy friend to a seminar which she invited me to, saying I didn't explain whether I wanted to go or not

She hasn't texted me in 3 days
She's nearly 20 and to go from such proclamations of love to falling for another guy in 3 days seems very immature and unlikely?

It means they're full of **** & don't want you to cry.

So she has unrealistic standards?
I just struggle to come to terms with the idea that, if |I'm a good boyfriend, why dump me? And this is why I find it hard to take her words at face value; just feel kinda nexted.
I'm friends with ehr due to the circumstances of the breakup but so many mixed messages. Hasn't contacted me in 2 days

She's stringing you along now. I'd break contact with her because it will be easier for both of you to move on. She probably just wants what she can't have and is really confused.