This is easily salvageable, you've got 6 months to go.
Speak to your DP tonight. Maybe show him the OP. Say you're feeling overwhelmed with it all and it's all got away from you a bit, but you're not happy.
Weddings do involve a lot of compromise, but there are some quick wins here.
Firstly, see if you can return your dress, and if not, sell it. It needs to bring you absolute joy. Go shopping again and get a new one. Dresses can have up to a 6 month lead time, so I'd do this one first. But you still have time, absolutely. Go with your friends or close family, choose something that is absolutely just for you and that you love.
The church thing is probably harder to fchange now, and your DP doesn't want a church wedding. Is a blessing afterwards something you could compromise on? You could go a few weeks later if your priest is amenable? I have friends who have done this.
Flowers - nothing is set in stone yet with 6 months to go and I'm sure the florist can redesign something within your current budget. At the very least you need to redesign your own bouquet because again, that's yours. There is surely a middle ground between what you and your DP want. MIL's views shouldnt really come in to it I don't think.
Colour wise, probably harder to change now. But could you tone down the purple by getting all white flowers, for example?
Talk to your DP, he will probably be mortified to realise you feel like this and want to change things so you feel better.