Is it normal my parents constantly ask me for money?

My parents are in really badly paid jobs so they struggle to keep up with bills and pay their debt etc. They both have had countless opportunities and offers to take courses to get qualifications and to go to university so they can get better well paid jobs but they have turned these offers down because it will be 'too much effort'.

I became an independent adult at 17. I've been paying my own way through life for four years. I've had a ton of shitty jobs. I go to uni and will be in about £40,000 worth of debt when I leave next year. I have a really intensive course. I spend hours in uni. Out of uni, I spend hours doing reading, notes, revision. I also have to do unpaid volunteering schemes a few hours a week. I have to do hours of shitty unpaid work experience just to make me eligible and attractive to employers to give me more intensive work experience in the summer and then someone may offer me an actual job. Outside of this, I have to find time to do shitty bar work, sustain relationships, eat and sleep.

Money is extremely tight at the moment. There's been times where I have gone three days without even eating because I could not afford food. I'm overdrafted to shit. I've done everything I can. The other day I even considered having sex for money because I'm so desperate. I blackmailed someone for money a few months ago. I'm turning into a shitty person. My parents know how much I struggle, yet today they asked me again for a large amount of money so they can 'buy a present for their friend'. I was sat there in disbelief. They know how poor I am yet they are asking me for money so they can buy presents. A few months ago, I asked if I could borrow £10 since I didn't have cash on me. I got accused of being rude, cheeky. They said I was disgusting for asking them for money and they didn't speak to me for an entire week.

It's really bringing me down. I'm really stressed out and over worked. I tried talking to a friend but it didn't help. I don't want to tell my boyfriend. He makes a huge amount of money in what he does but I don't want to be a burden on him. He would really worry if he knew the situation I was in and it would make me feel guilty and dependant if he tried helping me out financially. I really wish I had a family that didn't do fucked up shit all the time and not only contact me when they want something from me.

Is this normal? Because I don't know anyone else with inconsiderate parents like this. I even feel extremely guilty calling them inconsiderate.