I didn't mention those, because you were saying it was because you were offended that you weren't high enough on their list to be invited in the first round.
Not being able to afford to go is a valid reason not to. And while I think coping with being newly single is something you'll have to learn to do, if you don't want to do that at the moment, then that's Ok too. But those have nothing to do with the way they invited you. You are perfectly entitled not to want to go, for whatever reason. I think it's better to accept that you don't want to go, rather than trying to find a justification for it about not being invited first round, because the more you focus on that, the more you'll come to believe that they did something wrong, it was rude, you were hard done by etc,- you'll just subconsciously take that view on, and end up feeling more aggrieved. And it will show in the end, and you might then get fewer invitations to things at a point when you might feel like getting back out there.
So don't go if you don't fancy it, don't want to spend the money, don't feel like having conversations with people. It's a perfectly valid choice all on its own, and you can accept that you are in control of deciding whether you fancy a party or not without feeling you need to explain that somehow to anyone, including yourself.
I tend to accept invitations that I don't really want to go to, partly because I am insecure about not being invited again. I'm certainly not top of anyone's list, and I don't particularly expect to be high up there, so you're obviously doing better than me there. I'm more grateful to be included at whatever point. I'd love to be more interesting and exciting and the sort of person that people always think of when planning a guest list, a 'unmissable' party person, but frankly, I'm just not and never have been. If that's been your role in the past, then there's no real reason why you won't get back to that once you feel ready to go back out again and be social.