As a coach, whenever a gymnast comes in with a date that they must have something, I always groan. It usually means that they don't care about doing the skill correctly, just want to do it. Because of that, it often takes them longer to learn the skill than they had hoped. As you are trying to kick over off the mats, take your time and try to do it correctly -- push those shoulders over your hands as hard as you can, kick a nice straight leg all the way over to the floor. Don't let your leg drop back down and don't pull the second leg up too soon. Lots of gymnasts who are getting desperate for this skill kick the second leg up before the first one has hit vertical. This ends up pulling you back down to the floor.
A good stretch for your shoulders that you can do yourself is to put your hands on a surface at about hip height if you want to stand about 12 inches if you want to kneel. Keeping your arms straight lower your chest down towards the floor and try to get your armpits on the floor (if you're doing it standing, that obviously won't happen, but the principle is still the same). If someone else is around, they can put a hand between your shoulder blades and give a gentle push. Good luck!