Hey gang,
My practice seems to have hit a wall.  I have experienced samadhi perhaps a half dozen times, always accidentally. The last time I did it was many months ago, and ever since, I keep comparing that peak experience to my current practice, and this comparison makes it inaccessible.  I know I must let the past go, and realize that piti and samadhi I have experienced are just causes and conditions, but I seem to have issues judging my practice, which concretizes the sense of a self/meditator and traps me in the past. 

Does anyone have any advice on how to let go of striving for previous highpoints in meditation?  My teacher says I'm in a Vipassana phase, and am therefore unable to experience piti because my mind is grasping constantly, but I don't know how to let go.   

what is your teacher advising you to do?

The point of meditation is to help you stop striving and clinging. If you could just stop doing it you wouldn't need to meditate. So get used to striving and clinging, you won't get rid of it until you have perfect enlightenment.

My advice is to meditate first to relax and quiet the mind and then to do some kind of vipassana practice.

I think the link on relaxing meditation will help you the most with the question you are posting about, when I am fully relaxed, nothing bothers me.

This might be interesting for you too:
https://ncu9nc.blogspot.com/2020/10/a-quick-guide-to-producing-bliss-with.html

Also, how do you define samadhi, I've see the term used in various different ways I'm not sure what you mean here.

To give up on feeling pleasure or gaining anything.  He recommended relaxing the body and mind and then lingering in mindful presence while observing thoughts, and also choiceless awareness/nirvikalpa samadhi and gratitude.  He also said this sort of downturn is totally normal.

This is very much what my teacher suggested as well, so it's comforting to hear there is a concensus. 

EDIT:  I read yor links... thanks very much.  This is wonderful stuff.  I do yoga a bit before my evening sit, and look forward to trying some of these relaxation techniques tonight.

I define samadhi in the sense of access concentration/one-pointedness... an effortless focus on the object with the accompanying piti.

I think a problem I have is I don't know how to let go right now.  I was doing it just fine till I made some progress.  I thought, "Wow!  I've learned how to do this reliably now."  But the very next day I expected it to come and it never did.  That was back in October, some five months ago.  What changed is I now have something I am trying to do, I feel.  Before, I was just interested in what revealed itself naturally.
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" What changed is I now have something I am trying to do, I feel.  Before, I was just interested in what revealed itself naturally."

Nicely said!

Of course, we can also say that everything that arises reveals itself naturally, as there is nothing that is unnatural, and trying and not trying are both natural conditions. My first access to strong jhana was after spending a whole weekend trying. The last sit of the last day was only 20 minutes long and, knowing that would not be long enough to achieve anything, I gave up trying. You know what happened next. But I don't think that anything would have happened had I just sat down for 20 minutes at the beginning of the retreat and not tried.

​​​​​​​There are a lot of conditions involved, one of which can be having a good teacher, and it sounds like you've got that one covered. 

Seems like pretty good advice. I would say you don't really need to "give up" on feeling pleasure or gaining... just don't expect every sit to be pleasurable/productive.

There are always going to be difficult sits or sits where you feel like you are going backward... but those kind of sits always lead to much better things over time, it's part of the process. Just gotta be patient, lower short-term expectations, and stay committed to long-term progress. 

I remember experiencing this same struggle in the first years of my practice. The dynamic one is stuck in, where the desire to achieve samadhi, and the excessive effort one expends on its basis, actually makes it impossible to achieve, can be an instructive experience. This samsaric pattern plays out in the lives of people in countless ways. E.g. one aspires to become a doctor, but through a narrow-minded focus on this goal, one neglects one's mental health, and so burns out over the course of one's training. The grasping at certain states, founded in a reification of concepts in one's mind, is the fundamental problem that one aims to overcome through one's dharma practice, so there's no straightforward answer to how to stop this. It's so so great that one is engaging in the attempt though. Huge respect for that.

One rule of thumb is that one makes progress by uncovering and seeing through the mistaken assumptions that inform one's decisions about what to do. You are perhaps assuming that samadhi is necessary for making progress in your practice, or that you need samadhi to feel good and achieve peace of mind, or that one's natural state isn't itself a kind of samadhi. 

I think your teacher is right to advise vipassana. One can proceed by noticing that undesirable state A (ordinary distracted mind) and desirable state B (samadhi) possess the same underlying nature (e.g. share the 3 Characteristics as described in MCTB ), as do all states, and really letting that sink in over time, so that one develops disenchantment with the idea of attaining samadhi. When one really knows in one's heart that samadhi and non-samadhi are of one taste as conditioned and completely illusory samsaric states, then typically one can drop into samadhi easily without it feeling like a big deal.

You can try varying the conditions, i.e., time of day you practice, your posture and so on.  It can help with striving to be in a more relaxed posture or a time of day or mood where you're less likely to be drawn into striving too much.  

Also no big deal, your brain is figuring out how to concentrate and it will self regulate if you just stick with it and have faith and keep practicing.