[Solved] I don't know where to start. . .

Apologies if this is in the wrong place, or if its a bit of a rant. This is my first post.

Firstly I have 2 children from a failed marriage. They are 6 and 5 years. I have residency of the youngest, the ex wife has residency of the eldest. But I have both kids Thursday to Monday.

2 years ago, I met someone new. She informed me from day one that she didn't think she could have kids. I accepted this.
Soon into the relationship, it became clear she had trust issues with me. I remained friends with a couple of ex's. She really didn't like this. I also have a lot of female friends. Whenever I spoke to them I was accused of sleeping with them. I was just about to throw in the towel, when she announced she was pregnant. So I thought I would ride it out and see what happened.
Throughout the pregnancy, things got worse. Every time I did anything involving a female, I was cheating. I phoned the speaking clock once, and she came screaming at me demanding who it was I was sleeping with, and ripped the phone out of my hand and started hurling abuse down the phone. Just because she heard a woman's voice.
There are loads of other personal things, but not sure if they are all important.

This pregnancy wasn't without worry though. Her eldest sister died of a blood condition that was aggravated through pregnancy. So the docs were keeping an eye on the ex.
All was lumpy and the little fellow was born after an emergency caesarian.
All was ok for 5 weeks, then all of a sudden, she ended the relationship, claiming that I wasn't supportive enough in the delivery suite. Because of the blood condition, her other sister was very worried, and I was keeping her informed by text of what was happening. The phone was off when pushing started, and was only turned on when everyone was ok and out of theatre.
And now the problems start. She is convinced that there was someone else while I was with her. Despite me proving otherwise numerous times. She keeps getting verbally aggressive with me, and every message from her via facebook, text, email or msn is aggressive. She is making it her plan to stop me having any access to my son. Her main argument is that I already have 2 kids, I don't need that one. Trying to get information about my son out of her results in heated messages for several hours before I get 'he's fine' from her.

When I do go to see him, she spends the entire duration of the visit shouting at me, insulting me, and on a couple of occasions, physically abusing me.
She wont let me bring him home because she wont trust me with him. Because on one occasion I dipped the tip of my little finger into my coffee to put on his tongue. Something all of us have probably had done to us as babies. A couple of days later, I tried the same thing with gravy. Her mother (I forgot to mention, she still lives with her mum despite being 36) screamed at me.

It is getting to the point where I just want to walk away. I do have 2 other kids to think about, and all this is depressing me. Most people say she is not mentally stable, and I should go for full custody.

But now she is currently in Hospital. Her blood condition is playing up. It is serious stuff. I offered to look after my son while she is in there, but she refused saying he was with her in Hospital. Then 5 minutes later, she is asking her sister how he is. Turns out the sister has got him. I have been told that there is nothing stopping me legally going to the sisters house and getting him. Is this right?

Would I be wise in going for full custody of him?

I am also considering going for full custody of my eldest soon, as there a few behavioural problems that his mother lets slide. I don't think its good for the kids to have such an even split between 2 sets of rules. Should I have the 2 kids from ex wife for a longer period where I can install proper discipline with them?

Note: My name is on the birth certificates off all three kids.

Again, sorry if its in the wrong place, inappropriate, or a bit of a rant.