My wife doesnt show affection

We are married with 1 child 9 years old, been together for 18 years, I’m 44 and she is 38, I have always instigated sex and over the years I have realized that it’s always me doing the trying all the time. My wife doesn’t kiss me, no cuddles, no affection and I am just so fed up. I can’t speak to her about this, a few years ago I got a lecture about how sex is all I want. She can’t switch off from paying bills to enjoying sex, I can but without sex, I am starting to resent her every day. I can’t figure out why she doesn’t want sex and affection, it’s eating away at me. I stopped instigating sex 4 months ago and we had no sex, she has just ignored me in bed and we just carry on just living and surviving. I know she isn’t seeing anyone else and she doesn’t go out much. The other night she went out with girlfriends and came home merry and she instigated sex, in my head all I wanted to know is why haven’t we had sex for 4 months, we had sex and it was great, the following morning nothing again, she is always nasty the next day and it feels like she regrets being nice. I just need to hear from anyone who has experienced this or can give me some advice to get back on track. I don’t have anyone to talk to.