In short, no, you’re not a bad mother. From what you’ve said above, you walked away from an abusive relationship which can only benefit your son’s wellbeing. That’s not bad parenting, that’s putting your son’s welfare first.
As for feeling guilty that you can’t afford to go on holidays and trips with him, that’s a normal feeling that a huge number of single parents feel, myself included. Again that shows how much you care for him as some parents wouldn’t even be bothered. I was once told that all you have to do with your kids is love them and play with them. Whether you take your son on holiday or not, as long as he’s happy that’s what he will remember as he grows up. Simple things like going to the park or going for a walk and exploring will mean just as much to him. Try not to look at what others have and focus more on what you have and can do. It takes time but you will get there.