Nail Trimming: It sounds as though you need to start all over with nail trimming so you can turn what your dog does not like into a positive experience for both of you! Every day you should bring out the nail clippers while your dog eats and practice handling his feet and tapping his nails with the clippers while you feed him a tasty treat. It can be helpful to have a second person to feed the treats while you tap his nails with the clippers.
Once your dog is comfortable with his nails being tapped with the clippers, start thinking about what position you want your dog to be in while you trim his nails. Many people like to hold a small dog, lying on his back, on their lap. Large dogs can lie on their back or side between your legs on the floor. If your dog fights those positions, just choose a position in which he feels most comfortable. It also can be helpful to put small dogs on a table on a non-slip mat and large dogs on a bench or chair so you can reach their feet more easily.
Lure your dog with a treat into your chosen position and reward him for staying in this position. A “lick-able” treat, like peanut butter, is an easy way to reward in this position. Now, start handling your dog's feet and tapping with nail clippers while in your chosen nail trimming position. Once your dog is relaxed with this process, you can start clipping one nail per session. Take care to trim only a very small amount so as not to cut the quick. Gradually work up to trimming all nails in one session.
Harness: It sounds as though your dog is really excited to get his harness on so he can go on a walk. An excellent way to conquer this is to use treats to reward your dog for staying still until you get the harness on. It can help to have a second person to assist you with this until your dog learns that staying still gets him rewarded. Use a “high-value” treat – that is something your dog really likes – and start by feeding him some treats before he sees the harness. Bring out the harness and if he starts to get really jumpy and out of control, put it out of sight again. Every time he calms down, he gets rewarded – the entire time until the harness is all the way on!