I can see why you might worry about not being the father.
As the child of a sperm donor, I can tell you, my dad is and always will be my dad, regardless of the fact that he's not my "biological father."
My experience has shown me that parents aren't "zero-sum", if that makes sense. Learning there was another father-figure into my life changed, but didn't diminish, my relationship with father. Both my dad and my biological father represent critically important pieces of me and the fact that they aren't the same person doesn't diminish how I feel about either of them, it just means I'm the product of the combination of them (and my mother, environmental, etc.).
I can't know for sure, but I feel like what would alter how I feel about my father would be if he tried to hide from me who I really was (i.e. didn't want me to know that I was the child of a sperm donor). While I can understand wanting to keep that a secret to protect a child, I think it would have felt like the opposite if I hadn't been told. The fact that I'm a donor child feels like my secret to keep if I want to, not anyone else's - I'm grateful my parents told me (when I was about 10 - I'm now 38), and I'm grateful they've let me decide what I do with that information.
It's also worth considering that there's a good chance your child will eventually find out, even if you don't tell them. Whether through the prevalence of DNA testing, needing to know their medical history, or someone sharing information accidentally, it's something probably best told to someone in an intentional way - not happenstance.
The last thing that I'll mention is that when you choose a sperm donor, you'll likely have the option to choose an anonymous or non-anonymous (someone that is willing to be in contact with the child when he/she becomes 18) sperm donor. I hold no resentment towards my parents for using a sperm donor to conceive me (I wouldn't be here if they hadn't :), but I do wish they could have used a non-anonymous donor.
Like I say, my biological father is a part of me, but I don't know who he is. There are big questions for me (and my wife, and my son) about who I am, and until I find my biological father (still looking), I won't be able to answer them.
If you can, so your child isn't left with looming questions about who they are, I would highly recommend using a non-anonymous donor.
All the best to you, and your family, in your journey.
(throwaway account)