Yes indeed, it does happen and quite easily.
I think a lot of the "FIRE blues" comes from thinking about time lost. You read so much FIRE literature (especially guys like MMM who are very enthusiastic about it), and you start to think, "Every day I spend working is a day I don't spend living. I have to get out NOW!" Good for energizing you to take a first step but very very very bad for when it comes time to actually, you know, live out a ten to twenty year plan. You really can't wake up every morning feeling like you have to GET OUT NOW else you'll jump off a building before you reach year three on your plan.
I always try to keep things on the Maslow Hierarchy. Worry first about addressing foundation needs (food, shelter, health, etc.), then worry, but worry less, about addressing the upper level needs as you are able. Unless you are born rich, you WILL have to work a (relatively) long time to FIRE. Ten years, which is probably an optimistic goal for most folks, seems like a long time just starting out. But keep it on the hierarchy; you know you are working to meet the need, you've done the math, the plan is in place. You ARE getting out. It'll just take time. Meantime, don't worry about FIRE. It'll take care of itself as long as you stick to the plan.
The other thing that can help to beat the FIRE blues is remaining open to career changes. This can't always work for everyone, but if you've got flexibility, then try to actually have a career. Apply to other jobs that fit your FIRE plan and will give a shake up to the day-to-day routine. This will help you actually love your work since you'll have more control, you'll be learning more, and you'll gain more experience.