It's a difficult one - starting over after an ex who was shit to you in some ways but the sex was good.
I sort of had this with my exh, the only.part of our relationship that was consistently wonderful was the sex part.
Added to that, sex was part of our abuse cycle, when we were in the honeymoon period the sex was very intimate and the feelings we're highly addictive. Not healthy but that was how it was. Fairly common in abusive relationships tbh. It creates a strong bond and it's extremely confusing and upsetting when it ends and you still have good memories of the sexual stuff.
In my case, my ex was quite erm adventurous as well and we did things that my current pretty vanilla dp would struggle to even see the point of 
My current dp was actually pretty awful in bed when we got together, for a variety of reasons including having been in a lrt with a woman who hated sex. I can remember literally stropping off out of his house after he was selfish in bed one night.
Now years later we are very in sync and I look back on exh and realize some of our sexual relationship WAS actually also abusive but at the early stage, there were times I felt as you do now.
Abusive relationships really fuck with your head. They take good innocent things and make them addictive and upsetting. In a way, they can even cause you to find "normal" relationships boring because normal relationships don't have those horrific lows to complement massive highs! Including sexually.
This new guy, he didn't hurt you did he? He just sort of struggled to finish and it got boring and maybe uncomfortable? If so I think that can be normal in the beginning when you're shagging a lot but simultaneously finding your feet / getting comfortable with each other.
Not sure what the point of my post is, but I just wanted to let you know I understand how you feel and I don't think you need to be very alarmed by feelings that are pretty valid tbh.