This must be a terrible shock to you OP, and speaking as a guy, it's one of those 'worst nightmare' scenarios that any father would dread.
I think there are 2 separate issues here; your feelings towards DD, and the financial issue of supporting her (both in the past and future).
You've already said that your feelings towards DD won't change and you love her regardless, and I have to commend you for that. Some people may think that's an easy and natural statement to make when you've fathered her for 12 years, but given the huge shock and the impact of what you've just found out, I would argue that it can be difficult for some to see clearly. I know of a local guy who found himself in a similar position to yourself and virtually cut all contact overnight with the child who he thought was his.
As you know, many children are fostered, adopted, have step-parents and other loving relationships with adult figures/mentors in their lives - none of whom have any biological link. There's absolutely no reason you can't continue being Dad for your DD.
Regarding the financial issues, I'm very much of the opinion that your ex really needs to explain herself and there should be some legal retribution of sorts here. She has lived a lie for 12 years and carried you along unwittingly as a passenger in the whole facade. And on top of that, even if you HAD been the biological father, by the sounds of it she has completely abused your financial contributions which should have gone only toward's DD's wellbeing.
It's too late to change what's done now, but for anyone else in similar circumstances I would urge that you ensure your monetary contributions towards your child's needs are being spent on the child - not on holidays or booze or anything else. Insist on receipts. Use a specific account for that expenditure to easily track it or even order the items yourself. You can't account for every penny but you can minimise the risk of it being abused by a greedy parent.