How do I get my ex to realise it was depression's fault for disinterest and not mine?

What's interesting to me is that it's your ex you're making this clear to. I think the question you should be asking is why does it matter that my ex knows this? That point aside, depression can be a hard thing to understand if the person hasn't experienced it. A straightforward, direct statement that it was the fault of depression and not your own goes a long way in communication. It might not be possible to communicate such feelings to him when he/she hasn't experienced themselves, but as of right now the best you can do is simply tell him what you wrote here and hope they can understand.

I always find communication is key in almost any form of relationships. Maybe it's a good idea to completely explain to him/her how you experienced your depression and how it affected you and your relationship. (If you feel okay opening up to him about that.) That might give your ex a bit more understanding to the whole situation and also a bit of closure. Sometimes it's very hard for people who never experienced any form of mental illness to understand how strangely it might affect us. So my advice: honesty, communication and a hint of understanding, might do a lot more than you expect!

When it comes to dealing with symptoms of depression we want to be careful not to excuse our behaviour by reducing it to any particular diagnosis. It is perfectly acceptable to explain the cause of the behaviour by referring to your depression but be sure to recognise the impact your disinterest had. It's important to validate your partners' feelings in a situation like this. This way, they can have insight into the cause and have the reassurance that it wasn't because of them but they also receive your understanding alongside it. It's always good to find ways to address the issues happening I'm future, so perhaps you could map out an action plan for future instances to prevent it causing any further harm.