I had a similar thing with my mom and her (now) new husband.
the thing in my case was that my mom started dating (nice for her no problem with that) but completely became like a teenager in love about it. Which meant she dealt with her relationship in a completely non-mature way and quite disrespectful way (to the rest of us) which did give me the rage, and yes I was an adult when she started dating him.
Things that would drive me crazy:
1- in the dating phase, she would leave the house in the middle of the night to go see him without letting any of us know (i would just see her by the window or something). Nothing wrong with her going out, but let people know I had young siblings and I felt it was endangering them to not let us know that she was leaving so we (older kids) knew we were in charge of the youngers, and so in case of fire or emergency we didn’t waste time trying to locate her in the house thinking she was there.
2- She imposed him upon us (he made no effort with us) and stopped knowing how to behave decently around us. So for example first time we met him (he was introduced as a friend), he spent the night (we all got to hear!) and he then pretty much moved in from then on. After that, hearing them having sex or seeing gropping became an almost daily occurrence (even having my family over at Xmas didn’t stop them from touching each other under the table like two teenagers in love) and feeling like we now had to share our home with a stranger that couldn’t be arsed with us and a mother that was acting like she was 15 and Childress became the norm.
3- I felt she lost her identity and only came as a pack. If you called her to ask her about something she would always respond in « we » (her and him), and we did stop having time with just our mom. We just couldn’t do anything without him joining too. (Again if he had made a genuine effort it might have been fine but it was just plain awkward.)
Honestly her relationship put a lot of stretch upon our family unit (I already wasn’t that close to her to start with) but that’s because of the way she dealt with the relationship NOT because she was in a relationship. I just felt the way she dealt with things was completely disrespectful to all of us and it felt like her relationship was taking priority over even respecting our boundaries (we weren’t asking for much just « we don’t want to hear or see you be sexual, ideally, and it would be nice if we could have some quality time where X doesn’t have to be there so we can re-connnect as a family).
They got engaged and married and I know she waited to tell me for similar reasons to you. Personally I was fine with my mom having a partner and even getting engaged and/or married but really struggled with how she did things. On the other hand my stepdad (her ex who I consider like my dad) started dating someone new and I had a completely different reaction because he dealt with it the way I would have expected my mom to deal with it and so I actually really enjoyed seeing him date someone new. In both cases I don’t have that much in common with either partner but the difference in behavior made all the difference to how it’s been received and perceived (by the whole family as even my extended family struggle with my mom’s husband and I think it does have a lot to do with how my mom has done things).
Your daughter can’t stop you dating, and shouldn’t, and you definitely shouldn’t consider ending your relationship with him because of her, but maybe make room for trying to see things from her perspective and see if there are things you could do that could help that transition. You could do a « I am an adult and will do what I want irrelevant of how you feel or how it affects you » approach which is what my mom did (and led to all of us kids feel alienated and dislike the relationship) or you can have a « I like this person and will be dating them irrelevant of your opinion on it BUT would also like to preserve our relationship and make you feel like you are part of the process and while your opinion won’t change the outcome it’s still important to me » approach (which is what my stepdad did and was a much better approach).Nobody expect their parents to ask for permission to date, but consideration goes a long way because relationships impact not just the two people in it, but also the dynamic of the home etc… especially in context where the person moves in or will move in etc….
I am now multiple years later and we all tolerate the relationship and new husband (and we even have a new sibling) but are still all recovering relationship wise with our mother from something that could have been done so much more smoothly and successfully.
Ironically I’ll be hosting my mother and her husband/family in a few weeks in my home
and also have a new partner and while I know my mom wouldn’t technically care one bit I am dating I also know she would be extremely uncomfortable if I did what she did to us when she started dating (having my partner come to absolutely everything we do, showcasing continuous PDA & have loud sex while I host them, allow my partner to completely ignore them &/or make things awkward while also having them join to absolutely everything, etc… and if they ever ask me to maybe tone it down or that she would like one on one time with her daughter, for once, letting her know to « deal with it and accept it» just because it’s « my home » and they are visiting me and «to go stay somewhere else if they don’t like it/feel awkward about it»). I of course won’t be doing that because I would find it incredibly rude behavior on my part (just like I did my mother’s) because
sometimes it’s not about the fact that the person is in the relationship that’s the issue, it’s the behavior. My mom was inconsiderate and alienating when she started dating and that’s what I objected to. If I did the same she would feel uncomfortable because my behavior would be inconsiderate and alienating, not because she cares I date.
If you and your fiancé (congratulations!) are considerate, and have been mindful of her, I wouldn’t worry and carry on as you are , if there are things you do feel might make her feel alienated or like she isn’t being considered (being considerate isn’t something you only do for kids, it’s also important to be considerate of others in general, even if they are adults and can technically/theoretically cope with more things) maybe try and see if there is something there you can both work on. In all cases her screaming and shouting is unacceptable and she needs to learn to express herself in a more mature way; and you do need to keep in mind that transitions are hard. Irrelevant of age.
good luck, I hope she warms up to your future husband and that you can maintain your relationship through it all.