You are not being unreasonable to feel sad and angry at the way your maternity leave has not lived up to your dreams, @Jelandguilty - and I'm sure you are by no means the only person feeling this way. Covid has taken so much from so many.
As @StylishMummy says, you need to grieve for the mat leave you didn't get - and, for me, that starts with accepting that your current feelings are valid and justified, and not beating yourself up for them, as you seem to be doing, judging by your first post - and your nickname.
It is only by spending time with your feelings, and accepting them, that you can move on from them. You tell yourself that it is OK to feel this way and then hopefully you will start to see that your feelings are not monstrous or terrible, and you will be able to see them as they are - normal emotions - and seeing them in perspective will make it easier to move on from them.
There will be so, so many things in your baby's life for you to look forward to and enjoy - years of them learning new things, saying funny things, doing mad, impulsive, infuriating things - there is joy in every part of your child's growing up. And just because the first few months have not been as great as they could and should have been, doesn't mean that the rest won't be amazing.
So you need to say to yourself that it is OK to feel bad about how your mat leave has been, and then decide to look forward to all the good things that are to come - things you can't even imagine right now, but I promise you are coming.
I was taught an exercise by my cognitive behavioral therapist, which I found helps me, and might help you.
Sit quietly, with your eyes shut, breathing slowly in and out, and focus on your breathing until you have relaxed. Then mentally picture all the bad stuff from your maternity leave, and all the negative feelings this has given you - picture it as a big, black cloud. Then, as you 'look' at the cloud, you can see that it is getting lighter at the edges, and you picture that lightness gradually spreading, so the whole black cloud is getting lighter and lighter, and the dark patch is getting smaller and smaller until it vanishes. For me, the darkness in my head felt like a physical weight, but this visualisation helped me to 'feel' that weight lifting - and as I practised this more, I found I could recognise the weight/cloud approaching and quickly visualise it lifting - which made me feel better.