Op, do you think it's fair on a child not to have a father from the outset. The statistical evidence is overwhelming that children do better with two parents.
There is also evidence to suggest that it is a secure and loving home that is better for children, not whether it is one parent or two. So a one parent home that is secure and loving is no worse off for the child than a two parent home that is secure and loving. It's broken homes that causes the most damage to children.
OP I am currently pregnant having gone down the IVF with donor sperm route at 35. I am under no illusion it's going to be difficult but I think many mums find it difficult, relationship or not. I've seen friends be let down by their OH for whatever reason, leaving all childcare duties to Mum. At least in my situation I don't have a partner to get irritated at for not helping out, my focus can be entirely on me and baby. I also think if you have been single for some time and are used to being on your own that it puts us in a stronger position to cope on our own. I can quite easily go on holidays/weekends alone whereas I have friends that struggle with their own company and always need to be in a relationship or around others. I expect they would struggle being a single mum more than I will and you will get similar people commenting on here that you shouldn't do it based on their own situation.
There are lots of books out there for single mothers by choice that I found useful in my decision to go ahead. I struggled with the selfishness for a while but the research I did put a different perspective on it and also prepare for the road ahead. I would recommend doing your own research/reading.
Ultimately for me having a child of my own is my one 'non-negotiable' in life and there was no way I was going to allow a man (or lack of) get in the way of that.
Only you can know which path you would regret more - child on your own or no child.
Good luck with the decision making - I think that's the hardest bit!