I agree…it sounds like you are over trained and need a solid 10 days or so of rest. Then when you come back you may want to restructure your routine so you can avoid overtraining in the future.
For the HIT I like it when I reach a plateau but do not have the symptoms of overtraining. I like to work it in for about 3 times a week for a few selected exercises. here is a few examples of what I do.
For benchpress I like “strips”
I start off with a warmup…usually the bar for 10 reps, then I add a fairly light weight (for me 155lbs)for 10-15 reps, then a little more but still light (205lb)for me for another 10 reps. Keep in mind all reps are controlled and no jerking with a somewhat slow movement.
Then I will load up the bar with a bunch of smaller plates that I can press for6- 8 reps and reach failure. For me i will have the weights in this order on each side 45,25,10,25,10. I will press that for 6-8 reps (failure) then have my spotter “strip” off the 10,'s then w/o any rest (I will keep my hands on the bar to save time) I will press the remianing weight till failure…and so on until I get to the 45’s on each side and I press that till failure for my last set. I only do one set like this for chest. I also think these are great for lat pulldowns since most machines have easy pin/plate set up.
You can also do this for other exercises…or try a more simple form of HIT and do single drop set…where you pick a heavy weight you can do for 6-8 reps till failure then drop down w/o rest to a lighter weight.
If the gym is pretty empty and you have the Dumbbell rack to you self you can try “up and down the rack” I like these for shoulder presses or curls. Start down the light end with the 10lb DB’s and do 3 controlled reps, w/o rest move to the next weight up…again 3 controlled reps. Do this until you reach 1 rep failure (all w/o rest). Then still w/o rest go back down the rack but this time do every set until you reach failure. When you get to the lower weights it WILL start to burn…just keep going until you cannot go anymore.
You can also try negatives…I highly stress you need a really good/strong spotter or spotters for these. There are a few variations as well. One way is to put a weight on you can press 6 time till failure (sometimes I do reps where a pause on the transition stage) and when you reach failure then you do 2-3 more reps where your spotter does 100% of the positive part of the rep range and you lower the weight (negative) as slow as you can.
Another way is to start out with a weight more slightly more than your one rep max and do the same thing.
One last form of HIT is very simple…it is called the rest pause. Simply reach failure and hold the weight for a few moments to get a little rest and perform another rep or two, with a spot if needed.
These are all to be used sparingly and I like to pick a few and do them for 3 week cycles. I will say the ones that I saw the best results were the strips/dropsets. And the up and down the rack …but havent done them in a while since it is hard to get a rack to yourself.
You may want to read up on overtraing and CNS overtraining…it is good to educate yourself on this to keep making gains and avoid injuries