what to say to an ex

Basically i came out of a two year long distance relationship (we saw each other 2 times a month max).

We split maybe a month ago and i’ve been doing fine without him i suppose but he messaged me last might saying the normal stuff boys do saying “i miss you and i want to fix things”

I’ve been thinking about it and i want to say something like “i can’t guarantee we’ll get back together but if you want to fix things you need to really try hard” but my mum said that sounds mean... if anyone has any suggestions on what to say to let him know that he needs to put in effort or whatever then please help !