About orgasm frequency and letting go (but not too much)

Hello everyone from a long time lurker but a newbie forum member. I've been using Aneros on/off for 7 years now and want to thank you all for great advices in this forum that have helped me during this wonderfull journey. Now when my life let me to have more frequent sessions again I have been wondering few things:

1. I have had something that I would a call super-o for maybe 10 times so far. I feel that "dull" sessions are very frustrating when I´m not archieving orgasm, although I get some nice feelings anyway. It seems that something is missing and those sessions are not worth of all that preparing process and I just end up laying on the bed feeling quite stupid. On the other hand, those times when I´m getting orgasm(s) are really amazing and beyond everything else. How do you proceed with dull sessions and do you get orgasms more easily by the time? For me it is impossible to tell beforehand if this time will be success or not.

2. The second thing is about letting go. I have understood that to be able to have super orgasm you must go with the flow and travel where your mind and body want to take you. When "letting go" and getting close to orgasm, I´m having very strong involuntary contractions which are very pleasant but often shoot the Aneros out of my ass. If I try not to contract so hard, I´m holding back my pleasure also. Sometimes I push my ass down to bed while laying back which prevents Aneros shooting out, but this feels somehow wrong because it prevents the natural of movement of Aneros. I have many models and I´m facing this problem with all of them. So, how do you let go but not too much?

p.s. English is not my first language but I hope you can understand.