Hiya hun first I would get your child the support needed of the government. They won't help due to his age but you really need to sort that out. Go to citizens advice and ask them to help you with forms etc.
Once you have this the stress will ease abit.
My ex was an arse but eventually it's caught up with him. I understand its frustrating that he and his new family live a high life. However, resenting him will not help your child, it's easier said than done but refocus those feelings against him and put them into you and your child.
I had to brake things down in my head. My daughter had alot going on and I was always like her dad should be here. Over time I realised he wasn't and instead of giving my full time to her I was anger with him.
Iv learnt that all my children will forever be my children. They see what dad does and doesn't do and that's enough for me to get on with things and help them as much as I can.
I now don't have any anger towards him. I don't care what he has or his gf I don't even get mad if they don't follow through with things they say they would. This is because they are teaching my children that they will let them down always and I won't.
Christmas and birthdays really don't need to be expensive. I have 5 girls we talk about their list and I always say mummy will do her best, but what's better then presents is that we get to have Christmas together.
I wrap everything from pjs to booble sets.
You have alot going on at once and it's best the separate them things as emotions can be caught up where they shouldn't.
Don't worry about the support you don't have think of the support you do have.
You won't be in the position forever either. Good luck and hop all gos well