No Gains? Drop the Fluff
If you're not happy with your progress in the gym and your motivation is lagging, don't add more to your workout plan. Instead, simplify. Take a look at how you've been training the last couple of months. There's a chance you've been overdoing it, either by having done way too much work or way too much shitty work.
If you're used to combining a lot of high intensity work with a high volume approach (lots of heavy work, lots of sets, reps, and exercises), you're heading towards trouble. This is especially a big pitfall when you create your own training programs, since we all tend to think we're advanced and can always handle more.
When stagnation hits and motivation lags, ask yourself: What can I get rid of in my training? What makes me better? And what am I doing that's just busy-work?
Simplify, Reenergize, and Recover
Your body is a system of systems that requires recovery from the stress you place on it. When you reach a plateau, or even worse, your performance regresses, it could be a training problem, but it's probably a holistic problem. There are things that take your stress beyond your recovery capabilities.
Figure out where the stressors are coming from. A simple and highly effective strategy is to make your training as minimal as possible. Sometimes doing the least amount of work will create the most amount of progress.
Consider the Pareto principle. It states that 20% percent of what you invest tends to be responsible for 80% of your results. If you look back on your training history, you can probably see this. There are certain strategies and exercises that are responsible for where you are today.
Dump 80% of your training, do shorter training sessions, and see what happens. Not only will you likely perform better, you'll probably start to build up your enthusiasm for training again since you'll have more energy.
If you're a powerlifter or athlete, it's not the biceps curls and side raises that have been foundational to your training. If your program is full of accessory work, remove all of it for a couple of months and see what happens to your performance. All that extra work may be overstressing your system via accumulated fatigue.
Stagnation is a Signal from Your Body
It's telling you to change something. If you don't, what do you think will happen? Certainly not progress. We do need to push the envelope from time to time, but not all the time. The Olympics aren't held every week, all year round. They're once every four years.
Have you ever considered looking at your own training with this kind of timeframe?