I feel your pain, I have a boy aged 4 and a girl aged 6 at placement and in those early weeks and months I honestly thought I was losing my mind. They both had very different personalities, different needs and different responses so what worked with one was absolutely disastrous for the other. I felt like I couldn't do right for getting it wrong.
My girl was very reactive and very dysregulated, had very limited language for feelings and was very suspicious of anyone showing her care because those relationships had never been safe for her. My little boy was very affectionate, much less developmentally delayed and in many ways felt easier.
You're getting help with the parenting part so I'll share what helped me cope with my own reactions. I had to regularly remind myself that I was parenting two children from a standing start, I didn't have the benefit of relationship with these two so building a good relationship with each was my primary focus over and above any kind of behaviour management. So I'd only focus on behaviour that was dangerous, my girl would be violent with her brother so I intervened there but from a place of safety rather than behaviour if that makes sense? We did a lot of work on feelings, and keeping both children close - my two needed a lot of physical contact and needed to know where I was, what I was doing and why so I'd chatter through my day "I'm just going to make a cup of tea, I'm right here, yes you can come too..." all through the day. Remember they don't know you and your routines and anything unpredictable can be very scary so over-talk and over-explain.
I also reminded myself that however hard it was for me, it was much much harder for them. which sounds very simple but it meant I didn't get annoyed with them as much, and could respond to them from a place of care. Someone here spoke about the children behaving in front of me, not towards me, which depersonalised some of the challenges and helped me think about what they were trying to communicate through their behaviour rather than thinking they hated me, that I had made a mistake etc. Your mindset is really important in connecting with children who for whatever reason might be hard to connect with - finding something you like and value about each child and talking to them about it, being explicit about what you see in them and praise, praise, praise.
Make sure you get time for yourself, to decompress and relax - it'll build capacity for you to keep going with some very hard behaviours and help you not lose yourself in it all.
The last thing I'd say which may not be an issue for you, but certainly was for me, is that I found myself gravitating towards the "easier" child. I found my girl's behaviour very challenging and started to pull back, or would take myself off when she was being aggressive or violent, which was absolutely the wrong thing. I learned she needed me close to her, needed me to be with her even when things were kicking off.- possibly more so. Me staying with her and trying to connect with her when things were hard really mattered to her and gave her a sense of trust that I wouldn't be going anywhere. And seeing the fear and pain behind the behaviour helped me fall in love with her.
Four years on and there are still sometimes challenges, but the relationships are very well established now and you literally wouldn't recognise my girl from the child who arrived here in the beginning.
Good luck with it all, you've come to a good place here, lots of very experienced adopters who have been there and found a way through and who are very generous with their time and support. You're doing a great job, mum.