Hmm. I agree with Holdmecloser. Don't raise this at this barbecue you're going to, it wouldn't be fair.
Fair enough talking about it with Ms.Spa, but do it over the 'phone, or by going to a cafe with her.
Also, if you are going to talk to her about it, you should do it in an open-hearted way, assuming the best. Seriously. You must be clear in your mind as to why you are raising it. Is it to punish her? To claw back self-esteem? To make it clear in your head why you are dumping her? These aren't really good enough reasons for such a conversation.
A good reason is to clear lines of communication that were opaque before. So ask her what was going on. Say you want to know the exact situation wrt the voucher, because you didn't think to ask before and it has left you feeling uncomfortable. Give her a chance to be clear.
And you will have to accept it if she does have a clear response.
Really, if you only want to talk to her about this because you are cross and hurt after years of being made to feel second-class - and you just want to communicate that - don't do it at the barbecue!!
Apart from anything else, it'll ruin the occasion for you.
Really, you are doing all this work on raising your self-esteem, so start thinking hard about how to make sure you act in ways that enhance your life, and really do treat people like this in ways that diminish their importance to you and their effect on you.
Good luck.