I would echo the advice "Take your time". The car will be going so quickly that the gear change does not need to be rushed. Stay relaxed and change at your leisure.
When my pupils struggle with going into the correct gear, I advise as follows:
The gear selection is quite narrow and it is very easy to push the gear stick too far, or not enough, when changing gear. If you push the gear stick out of 4th a little too far, you end up in the channel for 3rd and then, when you try to move across into the channel for 5th, the gear lever can't do it and so you end up in 3rd. What I recommend is to push it out of 4th and then release your grip on it for a split second i.e., let go of it completely. This will allow the gear stick to find its natural resting place which is right in the middle. Then, take hold of it move it to the right and then forward. This should mean that you do not end up in the channel for 3rd.
A couple of things though. Don't take your hand away from the gear stick; the release of it should be for a nanosecond only. Follow the diagram accurately and think of it as 3 separate movements: forward, right and forward again, rather than one single movement. If you add the little release specified above between the forward and the right, this should solve your problem.
Finally, relax when you do it. 5th is very forgiving and you can really take your time...
Best wishes