I have an 18 month old son and could have written some of your post. He has given up napping. He needs to nap. He shouts, hits, bites, gets frustrated, follows me EVERYWHERE desperate to be on me, refuses bed, has never slept in a cot and is very, very hard work.
I have an older child who was a dream to parent. No tantrums, no violence, no frustration. She was and is the easiest child in the world. I parent them both the same, I am happily married, nothing has changed. It's personality I think. I'm just trying to reassure you it's nothing you've done I suspect. You're probably doing your level best in the face of a demanding toddler's whims.
DS needs constant management. I genuinely think he's frustrated most of the time. He wants to do things he can't, be things he isn't and join the world in a way he isn't ready for.
I bet you have moments- as do we- where he takes your breath away with how kind he can be. DS will tidy up, clean, give cuddles, stroke us, sing, chatter and play. But it depends on him being well rested, well fed and on the wind blowing in the right direction. Other times and I could quite cheerfully take up breakfast gin.
I am feeling shitty atm as I have a 5yo who needs one on one time with me and her little brother won't abide anybody but him being near me. She takes it all marvellously and reassures ME that it won't be forever.
It's crap though. It passes. I think once they're older, better able to communicate and perhaps sleeping better, life will be easier. Otherwise I can only recommend chocolate, tea and swearing vociferously when they can't hear.
Try tackling sleep first. It's amazing how much better good sleep can make the situation. For both of you. I don't know about you, but we've got tooth after tooth coming through and I think a lot of what we're dealing with atm is caused by that. He's getting back molars as well as his last two canines and the drooling and biting is definitely down to that.