I find David Goggins advice helpful in these instances, which I myself experience regularly at 55 years young. Basically, stop negotiating with yourself. Give yourself no option but to do this. THIS is what YOU do. THIS is YOUR new norm. If you give yourself an out–e.g., I’m just not motivated enough to do this–your brain will convince you that its ok and even rational to skip the workout.
For me, this all focuses on age: my brain tells me: WTF are you doing, working out for two hours every day and sometimes running afterward or even doing a second workout? You’re an old fuck, you’re gonna hurt yourself, you can’t and shouldn’t be doing this. I ignore it. This is what I do. I refuse the negotiate. Make your brain follow you, don’t let it convince you it’s ok to stop or skip or back off because that’s rational or smart.
In any event, this is how I deal with what you are experiencing, viz., by saying motivation is irrelevant. This is what YOU do, and nothing can stop you from doing so other than listening to your brain tell you that you’ve been doing this for two years, you need a break, you’re overtrained, you’ve been hitting it too hard, you need to summon up some new motivation, and so on. Fuck all of that. This is what you do.