So this is kind of getting personal, but I am wondering what others' experiences are with having sex? I still can enjoy it, but it seems to have a bad affect on my neck and then consequently my head. I feel wrecked afterward and very spacey and bit more dizzy and it feels like because of tight muscles in my neck and even upper back.

This even happens after doing things like light yard work and gardening.

After 2.5 years of neck pt and working on posture, I still can't seem to do any activity that involves working my upper back and neck even lightly without causing increase in symptoms. Why is this? Will I ever be able to do activity like this again without having such problems?

I experienced similar chronic neck pain. It seemed to be triggered by everything physical, including sex. I threw a lot of resources at it. I had this theory that the pain was caused by muscle imbalance, and the dizzyness was caused by imbalanced traps pulling on the brain stem. Some doctors/people agreed, and some disagreed.
I spent a lot of time trying to correct 'muscle imbalance', but every excecise and workout I tried would only exacerbate the problem.
One day I switched chiropractors. I found a guy who was trained in atlas vertebrae realignment. He also has his own x-ray machine which removes all of the guessing from his practice. I must say that I previously dismissed atlas realignment because of all the hate on the internet. But, after 1 treatment and a few follow-up sessions my neck pain subsided almost completely. In the last 2 months I've been resuming almost all of my old activities without any neck pain.
I still think pain is caused by muscle imbalance, and that the muscle imbalance caused by a misaligned spinal cord, for which the epicenter was my atlas vertebra.
This treatment should only require 1-2 'atlas realignment' sessions, and 3-5 follow-ups for maintenance. Fairly cost effective in my opinion, I pay $45/ session which last 15 minutes.

Hains, Your atlas realignment experience is extremely unique. The ligaments that needs to strengthen to hold proper atlas alignment take up to 6 weeks or more to strengthen/tighten. Many chiros charge $300 to $600 for the x-rays and assessment before starting treatments. One should plan on spending up to $1000 for a full course of assessment and treatment. Insurance may cover treatments but rarely cover the x-rays and assessment.

Interesting. I forgot to mention that my treatment wasn't an instantaneous fix, but took a few weeks of aches and pains to adjust to the realignment. I would think the recovery time would also be unique and dependant on the nature of the injury, thr amount of tearing, the amount of inflammation, and the individuals biochemistry/ability to heal. That's where supplements, prolotherapy and LLLT can be complementary.
That being said, I still believe atlas realignment is worthy of a trial for those who have plateau'd, because although my case may be unique, it's still a positive case study.

I should probably omit future discussion involving finances, I live in Canada, so I guess i'm speaking a different language regarding treatments, maybe somewhat irrelevant. I'm not too familiar with the American system.