I have this issue with my mother, though it is much better now than it used to be. (After years of me implicitly and occasionally explicitly making it clear that I'm going to draw and keep my boundaries, she has backed off somewhat.)

When she attempts a guilt trip or other sort of manipulation, I refuse to engage and move the conversation in another direction. When she asks questions I don't want to answer, my stock replies include, "I don't know," "We'll see," "One day at a time," "I'll let you know if I have any news on that front," etc. But I don't have a problem with small lies to keep her out of my business. Why haven't you called lately? Been busy at work (even if I haven't been). Are you seeing anyone? No (even if I am).

Occasionally I will have to just tell her that I don't want to talk about something, or that it's my personal business. If there's something that we continually argue about (like your dad thinking you work too much), I will just tell her every time she brings it up, "We're not going to end up agreeing on this. Let's talk about something else." Then I change the topic.

Just remember that however it feels, your parents do not have the power over you that they had when you were younger. So do not give them any more power than they actually have. Good luck. I have found in my own case that things have gotten a lot better over time.