Gained weight, everything hurts, getting old, bjj class seems tedious, rolling is frustrating. What can a 36 year old blue belt do to get excited about training again? 
Can you visit some other open mats in town?
Do some judo, or wrestling to open up other avenues of grappling.
If you are serious about getting some fire. Sign up for a tournament and pay for it now.
PEDs and watch vision quest once a week.
Set a realistic goal.
I was a 35 yr old blue belt that was frustrated. I decided I would only arm bar white belts and leg lock blue belts for a week. I was only slightly successful, but it improved areas of my game and gave me something to work for. Another thing might be letting people get into side control and just try to survive or escape.
Sometimes training is hard when things are not going well and you only focus on the submission. Focus on smaller more precise goals. It might be frustrating not reaching the goals, but less so than not reaching some unattainable ideal.
I agree with smaller goals at a time for sure. I'd personally make doing some weight and getting into better shape a priority. You will only feel better in the end. There's the whole hammer and nail thing too 
im 35 purple. I dont like to compete but i do anyway. Helps me get into great shape and give me a goal/motavation
32 Year old black belt.
Distance and other commitments keep me from training as much as i'd like.
Your first sentence describes my situation right now to a tee.
Usually, going to class and getting beaten down by a lower rank lights up the inside fire for about another week or two.
Also i do judo on the side 1x a week...it helps a bit.
judo helped my game as well, I started at 35.
41 year old purple, I agree with everything posted here. I like to "theme" my weeks. Pick something & just work on it all week. Every class, every roll. Drill it extra! Like it was stated above, it doesn't have to be a submission. Maybe your having a mechanical issue in a transition somewhere. Trust me that spark will fire back up
Call Stella. She can advise
ChokeTheFace - PEDs and watch vision quest once a week.
This! TRT home boy and find your spirit animal.
checkuroil - Call Stella. She can advise
This made me LOL
FLOW ROLLING with people I really like. Takes the pressure off and we can just act stupid. Maybe a little time off.
checkuroil -Call Stella. She can advise
You need to vacation at Montego Bay. There you need to find some guy named Winston Shakespeare, he will take care of you. He love rollin w guys now, only no-gi and people says his jiu-jits "is on the down-low" not sure what that means, like, "sick!" or something. That's the guy you want to see.
I am 40 and I have different goals with different people, ect.. I have a some really solid game plans that if you knew me your probally thinking "oh here comes this tired shit again" but it high percentage every time. I say concentrate more on what works for you so you have a soliOd game to fall back on . Thats your long game, short game is trying new things out that ya might find relevant to your long game. But the short game should be playful, creative, fun with no real expectations of victory. If it gets competitive, go back to what your good at.
Don't take it too serious.
Lower expectations
Little victories
Your not that old! I rolled with a 61 Brown Belt today, he's solid and he surfs.
Some stout guys on our mats. Overweight and technical, just not a lot of triangles and guard arm-bars
Roll high while listening to metal...or.....perhaps you never had a groove in the first place. I would start here:) visit a school in WA.
Thanks for the great responses
baby-silverback -41 year old purple, I agree with everything posted here. I like to "theme" my weeks. Pick something & just work on it all week. Every class, every roll. Drill it extra! Like it was stated above, it doesn't have to be a submission. Maybe your having a mechanical issue in a transition somewhere. Trust me that spark will fire back up
this is a great suggestion
I do it for 1 month at a time.
Pick a position (either fill a hole in your game or take your A-game to a higher level).
Learn what your instructor thinks is the primary strategy, the most common entries, bread and butter techniques from that position and what you opponent is trying to achieve when countering your position.
Stop trying to PROVE jiu-jitsu. PLAY jiu-jitsu!
Some time off might help.
You might also consider switching schools, especially if this plateau and lack of enthusiasm has been going on for quite some time.
36 is not that old, by the way.
51 year old purple , concentrate on what you can do not what you cant do and have fun . Get at least three days a week on the mat and vary the intensity level depending on your condition
58 yr ole Brown belt.
As you can realize from all the great post, it's all your state of mind.
To sum it up , i really like what Setree posted, "Stop trying to PROVE jiu-jitsu. PLAY jiu-jitsu!"