Catrin, the only thing I can add, because you got great advice, is that perhaps the problem is in your core strength or body strength relative to the body's weight. I know you have lost weight and you work out at the gym, but I've noticed in heavier cyclists that they struggle to start the bike and get on the saddle, and when they lose weight and gain strength they are up on the saddle in one easy movement.

Another suggestion, use a spin bike at the gym to practice some skills, when there are no spin classes going on, like standing up and pedaling.