menstrual cup tips?

I purchased a blossom cup from amazon and I'm trying it out for the first time this cycle. I know there's a learning curve and I feel like I've watched about 100 YouTube videos but I just can't seem to get the thing to sit right for long. Inserting it is not as hard as I thought it would be, although it can be hard for me to get the cup to feel completely open. I've just been wearing it sitting around on my couch and it'll feel fine for an hour or so before I suddenly feel it leaking. When I pull it out, there's definitely blood in there, so it was in correctly at one point, but it's like it shifts. Even when I'm doing literally nothing but sitting on my couch. Any tips?? Is this cup maybe just not the right size/shape/firmness for my body??