Wow, ok, lots of things going on here that I see. First, I would get a chiropractor out to see her as soon as possible. Maybe even have an equine massage therapist out.
So say the physical aspects check out. I will argue that her not standing still for mounting and being buddy sour and all that IS linked to a relationship issue. If she was completely trusting she wouldn't act that way. So, you need to figure out when she actually starts getting tense. Is it when you put the saddle on? Is it when you get a certain distance away from the herd? If she wants to get back to the herd she is telling you she doesn't feel safe with you....the herd represents safety to her. So now it's your job to prove to her that she can trust you and everything will be okay when she is with you.
Say she acts tense when you put the saddle on. Play approach and retreat with it....put it on, take it off, etc. until you see her relax. Do not have her tied for this, she needs to be able to sniff the saddle if she wants to.
Say she gets tense when you are leading her away from the other horses. Again, play approach and retreat. Go a short distance away then go back, go away then go back, etc. until she relaxes. Also make it a habit to take her out and let her graze. This will build rapport with her and she will have more confidence in you.
So say things are going well and you want to ride. If she is afraid of the mounting block start there. Again, play approach and retreat. Ask her to sniff it by sending her to it (not leading her up to it) and if she stops or hesitates (which is her telling you she has reached a threshold) allow her to assess the situation and then take her away from it. Send her back, take her away, send her back, take her away. When she finally sniffs it take her to go eat grass for a few minutes. Then go back and do it again. Be prepared for this to take time and be patient.
So now you have her confident in the mounting block. Now prepare her for you getting on. Slap your stirrup fender/leather against the saddle with rhythm to get her used to that commotion. If she wants to move let her. Go with her but have her head tipped slightly toward you so her hindquarters go away from you. Do NOT try to make her stand still. When she stands still AND relaxes stop and rub her. Give her plently of time to think about it and do this on both sides.
Now simulate your weight in the stirrups. Lean on the stirrup with your hand and if she moves, again try to stay with her. When she stops and relaxes release the pressure. Make sure she is completely confident with this.
When she is confident with that get the mounting block and step up. If you see her eyes get wide, her body tense up, her head go up, etc. step down then back up, down and up, down and up, until she doesn't care. All the while rub her. Do this on both sides. So now you have her confident so put your foot in the stirrup and take it out. Put it in, take it out, until she is relaxed. All the while rub her. Now put a little weight in the stirrup and take it away, etc. Work your way up in little increments until you can stand in the stirrup. Do not swing your leg over immediately, wait and rub her on the off side. Now you are asking permission to get on. If she is okay gently swing your leg over and rub her for awhile. Then get off. Maybe end the session right there. Don't rush this, take the time it takes.
Once you start riding do exercises to get her to relax. Ask for lateral flexion, circles, leg yields on the circle to release tension in her belly, etc.
One thing on lunging.....the fact that she seems to get more worked up when you do this is a big indicator she is not confident. I am a big promoter on working your horse on the ground before riding to see what kind of mood they are in so if you lunge her make it have a purpose. Don't just go in circles. Put out some poles, ask her to jump things, set up puzzles for her to figure out, make it interesting.