I have a plateau too, which my body normally hits BUT what I did when I first lost below 140 is when I was starting to get close to where I had set up this boundary for myself I started working out harder, and I picked up something new, whether it was adding swimming or biking, something a little different. It's good to lose the idea that you are going to regain it, and that you will stop there, but if you can't fully get rid of that idea, use it to your advantage. Change it into something positive. You know you have a challenge ahead of yourself, BUT we can all succeed in pushing past them if we decide to.
I remember when I was rowing (I rowed varsity level for 6 years in highschool). We had to do races on the rowing machines for top spots in the boats. when I was starting out I couldn't get below 8minutes for 2000m. No matter what I did I thought I'd never get below it. Then I had a competition coming up and I just decided that enough was enough and that I was going to break through this "brick wall" that I had set up in my mind. So I got extra excited for the race, trained a bit harder to give myself that extra self confidence, and I ended up getting 7:52.
We all set up boundaries, or I'd say most of us do. Just remember that you are fully capable of going below 150 if thats what you want to do AND you are more than capable to stay below 150 without gaining it back.
Plus I'll be there right with you because I think my body gets stubborn at 140.
** Oh I should say too that me encouraging you to go below 150 is me believing from what you said that you want to go below 150 and that it's healthy for you. **