Why do my parents never approve of my relationships?

I’m an only child to slightly older parents, who are (arguably) a bit protective over me and we’re a really close family unit. Usually they’re relatively cool with me doing things (especially since I turned 18 this year) but this has never extended to relationships. I’ve been met with strong disapproval about my past two relationships (although I can understand these people were not the best people ever and I quickly broke it off with them) but they’re also staunchly not talking about my current girlfriend, who is absolutely wonderful to me, even if she’s more introverted than me (which is what my parents are holding against her).

My mum is more open about it than my dad, but both are not exactly making it easy. I actually hid the relationship for 3 months because I was terrified of this kind of reaction. Today my girlfriend came over and since then my dad has been in a mood and my mum said ‘it’s hard for parents to accept their child’s dating’ which I think is kind of fair enough and I do understand that they may not like her that much but at the end of the day I really like her and it *is* my relationship and life after all, so I just think they’re being unfair to show how much they don’t approve and so put me in a position where I feel uncomfortable talking about her, inviting her over or seeing her.

I feel like I just don’t know what to do about it. Does anyone have similar experiences or have any tips to make things easier? I tried talking to them, which has been fairly successful with my mum but my dad is not reacting in the same way, and it seems to work better to just ignore it and not talk about it (it doesn’t help he also has a short temper and we often have to walk on eggshells around him). I don’t want it to drive a wedge between me and my parents and I care what they think, but I’m worried that if I focus too much on what they want and try to make them happy, I won’t be making myself happy and will be missing out on what is a healthy, stable and happy relationship.