You don’t necessarily try to motivate yourself to do the same weightlifting and treadmill workouts as before. If you’re tired of doing those things, you don’t have to do them. There are many ways to be fit that don’t involve weightlifting and treadmill workouts. It’s understandable that you’re not motivated to do them. Those activities have little inherent enjoyment. They are typically done strictly for the post workout benefits rather than the enjoyment of the activity itself. If the drudgery of those types of workouts is keeping you out of the gym, pick other workouts to do.
You might consider doing something like kickboxing classes. These aerobic classes use body weight to improve core strength. The same company that does BodyPump also has a kickboxing class called BodyCombat. With this type of activity, not only are you getting exercise, but there is an inherent fun factor. It’s like how people will go out dancing because they enjoy dancing itself. The fact that dancing also has a fitness benefit is a bonus. Classes like kickboxing can be like that. You may find them to be “fun” in some sense because the activity itself has attributes which makes it inherently enjoyable.
If you do try a new exercise, take it slow at first. Very likely you will be sore initially. Not only have you taken a long break, but you will be using muscles that you weren’t working out before. It will take some time for your body to get used to the level of exercise and and for the other muscles to get used to working out. Increase the intensity as the soreness decreases rather than starting out at full speed.
Some good points to consider. Another obstacle that I mentioned above is that if the replacement activity doesn’t have equal or greater week to week impact than what it replaces, then it feels like a regression and therefore a failure. Maybe that’s countered by the fact that I’m more likely to actually do it, but there has to be a line somewhere, right?
It really does sound like you are sabotaging yourself.
If it’s not hard to go to the gym to do cardio, do that. Just standing upright on the treadmill is some strength exercise, and better for you than sprawling on the couch.
And maybe buy some weights to use at home on a different day. Or find a class with weight bearing exercise that appeals to you. Or try kickboxing. Or learn to build rock walls on your property. (Mixing art with weight lifting.)
It will be a different workout, but you might find that you like the results better. A class might be a better aerobic workout since classes are typically 1 hour long. If you were only doing 20 minutes on the treadmill, then a class would probably bring more aerobic benefits. As for strength benefits, that depends. A class won’t bring the same results as weightlifting, but it may give you more functional strength. Classes typically involve whole body movements that create a strong core. Improvements in core strength can give you more practical strength for day to day activities, like carrying stuff. Weightlifting will produce big muscles, but they may not all work together in an optimum way. Doing full body workouts will engage many muscles in a synchronized way so that your body can be more rigid when it needs to be strong. Body weight exercises might make it easier to do something like carry a bag of cement compared to weightlifting. For instance, this “skinny” laborer is able is handle cement bags easier than huge bodybuilders: Bodybuilders vs construction worker
I personally gave up weightlifting for BodyPump because I found BodyPump to be a much more enjoyable experience. I initially did BodyPump because I wanted to do something aerobic on my rest days. I found the class genuinely fun and preferred to do it over boring weightlifting. Even though my muscle mass went down, I felt like my usable strength went up. My workouts became BodyPump, BodyCombat, and cycling. Rather than just doing a workout for the results, I would actually enjoy my time doing the workout because the activity itself was enjoyable. It made it much easier to work out regularly. Although I didn’t have the same results as pure weightlifting, I didn’t care. I was getting a good workout and having fun doing it. You might feel the same way. Or maybe not, and that’s okay. If it’s not working out for you, you can always switch to something that works better.