Hi again Juan - it's really important to find your foundation in this pose before you go up into it. Make sure that your feet are turned slightly inwards and press them firmly into the mat by your buttocks (spread your toes along the mat) and that your shoulders are square on the floor.
Tuck your chin gently into your thoat so that your spine is long. Don't worry about interlocking your fingers together behind your back if it's too hard - leave them by your sides with your palms down into the mat. As you breathe out draw in your mulabunda (pubic abdomen in towards your spine, so that your core is firm). Try not to clench your buttocks!

Before you go up take 3 breaths in this position - each time you exhale drawing in your mulabunda.
When you finally roll your pelvis up from the floor make sure you press your feet firmly into the ground and remember to draw your knees in towards each other (don't let them flop out to the side). Keep focused on making your breath as smooth as possible as your raise and lower your pelvis.
Often we try and push ourselves so that we do what everyone else is doing in the class, but it might not be right for us

. It's a hard lession! If you focus on keeping your core and foundation firm, you will, over time, start to open up so that you can go further into the pose when your body is ready. Hope this helps.