Let's break the ice with a hard fact: you're going to plateau. Your fitness progress will crawl to a halt, and it won't be a good time.
It might be two weeks from now or it might be two years, but it's going to happen. It's probably going to happen when you're feeling great about your progress, because that's just how life works. A plateau is just another curveball that you'll eventually have to deal with.
You need to do two things to break through a plateau. First, you need to figure out how to start making progress again. That's not an easy task and tends to take trial and error, but you can create a plan for it. Second - this is the real problem - you need to figure out how to stay motivated during a potentially discouraging time.
I get that it's not easy to create a plan for that, just like it's not easy to create a plan for any of life's crappy moments. But hey, that doesn't mean you can't do anything about it. When you find yourself in a progress-halting situation, try using one (or all) of the following tips to help you stay motivated through it.
Look at (or think about) your old workouts
I'm going to pretend you write your workouts down and track your progress like a good little fitness enthusiast, so hopefully you do. Pull out your training logs and find a time when you made better than average progress. Take a second to think about how awesome that progress made you feel.
If you're a normal person, you're going to experience a surge of instant motivation. Reflecting on times of progress is a great way to get your mind back into the game. There's also an added benefit: you have a chance to see what worked for you in the past.
Remind yourself that it's temporary
Plateaus are temporary, and you're not going to be stuck in one forever. It might feel like you will at times, but it's unlikely, especially if you stay focused on the task at hand. If you're the type of person that likes reminders, take a sticky note and write this down: it's only temporary. Stick it on your fridge or bathroom mirror or anything that you're in regular contact with.
Use your frustration as fuel
I know this isn't a good tip for everyone, but it's gold for some people. You can let your frustration wear on you until it subsides into the even more miserable form of discouragement, or you can make a preemptive strike. Use your frustration to fuel your workouts.
Plateaus usually require that you make a change in your program. I can't say what change will work best for you, but working harder is never a bad choice.
