OP -your thread struck a chord with me, not because I've been through similar, but just because you sound so unhappy about the situation.
You've been through a really truly difficult time. Having a baby is a huge thing that can really swing hormones around. And then you lost your mother. I so feel for you. People on here have mentioned PND. I don't know if this is perhaps a possibility. But a huge bereavement like that can cause anyone to feel depressed. You must still be grieving - the grieving process, particularly for someone so close, can take a long time. You might not totally be recognising how this might be affecting how you think about things.
I was also just going to mention that after having a baby some people can get OCD symptoms. OCD can feel very debilitating and it is probably not easy for others to always understand. I just mention it because you talked of the name feeling not right. Things feeling right or not right is something people with OCD might feel and may often talk about. If you had other symptoms of this it might be relevant both in terms of whether or not changing the name will help and also maybe in terms of how a partner may not understand the over-focus on certain things.
So - on the name front. Two things. You said at the start your DH didn't like the name Serena. I notice that you only talk in terms of these two names. I wonder if you've become fixated on Serena as being "right" and maybe also that somehow this "wrong choice" (of Cecila) has become associated with all the negative events you've suffered since. This is obviously not rational in that neither name is right or wrong as such and both are nice names. The fact you were dealing with a big bereavement at the time of first using Cecilia - when you weren't sure about it - may also mean you associate it with grief to some extent. But you might well feel the same about Serena if you'd picked this at that time.
It's a shame that a name you liked and still like feels so negative to you. I can't help but feel this must have something to do with everything that's happened whilst using the name. I do think that as you are still feeling negatively about it, that it might be an idea to speak to your DH and say you know it doesn't make sense to him, but you feel negatively about the name and you really would like to discuss it with him. Maybe show him this thread, which is full of people saying nice name but also that if you are unhappy then really perhaps you need to change it. He may not realise that you could be - perhaps - associating it with negative feelings due to circumstances and if he did realise this he might be more open to finding a solution.
However, in your opening post you said he never liked the name Serena and my worry is that if you feel that "things will be right" if you choose Serena instead, that you may be disappointed and move onto something else. You will still be grieving. What if you couldn't use Serena for another reason? Did you and DH have any other names you considered at all that you both liked?
He might feel a bit embarrassed to change the name now - having to explain it all. Or maybe it feels like a lot of hassle for reasons he doesn't understand. I understand this. But if you have a proper talk to maybe work through why you're feeling the way you are he might see that it's really not such a big deal to change at this stage. I'd explore whether there are alternatives to Cecilia you prefer that you both like with nn Cece so it doesn't feel like such a big change. There is Celia - which is shorter, as you said Cecilia was too big. There is Cecily which is slightly less formal. I really like Cecily and Cecilie and this name sounds slightly less formal and quite close to Cece? There is also Cecile which is a rather beautiful elegant name - good for an adult but shorter than Cecilia. It might be he would be more willing to agree to a similar name along these lines where you have the same nn but just "tweak" the long name?
If you look at those and don't like any of them or they all just feel negative due to being associated with the previous name, then perhaps it might give you a clue that you do just have negative feelings about the name Cecilia at this point - possibly due to going through trauma and grief at the very time you were first using it. In which case maybe the pair of you come up with something else entirely, together.
I would suggest - gently - that perhaps that name should be something different from either Cecilia or Serena then. Because the name Serena can't cure everything that happened and because your DH didn't like that name before anyway. Imagine your sister or someone had already used the name Serena - what would you use then? There is no perfection, and chasing it is probably not going to help the situation. There are loads of lovely names.
I think the key is really trying to explain to DH how you feel. And going from there.
Much luck to you OP. You have been through an awful lot and I really feel for you. I think you will get there and all these options - to all of us outsiders - sound nice. I don't think what everyone else thinks is the issue here. Let us know how you get on and be really kind and gentle on yourself. You've been through an awful lot in a short space of time. Things will feel better in time, but in the meantime if you want to change the name, I think talking properly with your DH and working together on it is the best way forward.