Sometimes when I date guys I like to picture what the future would be like with them
What kinds of things do you think about?
Finding a house, getting married, get pregnant, and then he would leave me for someone younger.
I think it is okay that you have those thoughts, but you shouldn't be so down on yourself. It's good to find a person you want to spend the rest of your life with. When you find the right one they won't make you think they will run off with someone younger.
Why are you telling me this? I don't really want a future with them.
It is okay if you don't want a future with them right now but you might in the future. It is good to have somebody next to you for the good and bad times for the rest of your life.