Dear OP, I am much older then you and I lived like this for a very, very long time. For me this started very early on, after our first child (we have three). Perhaps I had PD or some hormonal imbalance, but I personally think that it was all due to issues I had with him outside of bed. We were not matched very well and I suffered emotionally, mostly in silence for years. We finally separated last year, by the way, after being together for twenty five years.
I cannot explain why I just let it be like that. I did not question it, did not try to fix it, I just accepted it. Our sex life was all over the place, very irregular. I was not into it and I felt bad for my, then husband and now ex, so most times I would make an effort, but it was very difficult. He could tell of course if I was not into it and this would make him really upset, naturally. I had to try really hard, to make sure I am enjoying it as much as possible. I know this sounds ridiculous and I have no idea how I did it and for that many years.
Having sex when you do not feel like it and with someone you are not attracted to, for whatever reason, is very, very hard. It is like being raped every time, but you cannot really blame the person as you are doing it to willingly.
Please leave!!!! You are soooo young, your whole life is ahead of you. You deserve to be happy and satisfied in every aspect of your life. I wish you all the best.