IMHO 5ed 40K is a splendid game. The streamlining of the rules is very efficient, but each army retains it's unique flavour, and the rules allow for different tactics (even though mech is the easier way to go).
When it comes to your difficulties, I have some possible solutions, but first off: how much terrain do you use? More terrain that blocks line of sight lessens your opportunities to shoot him up before he can assault.
Now, perhaps you should practice assault denial. There are two ways to achieve this, and the first is to put as many squads as possible in transports. From your list I take it that you already put your tactical squads in rhinos and your command squad in the razorback - is that correct? I would get another Razorback, preferrably with the twinlinked heavy flamer, for your devastators, but not actually putting them in it. If your opponent is good player he will see this razorback as big threat to his forces and hunt it down, which is good, since that means that your troops can do their thing, and if he doesn't hunt it down, the razorback should be able to singlehandedly (wait, tanks don't have hands...) take out a third of his force.
Next step in assault denial, if possible, deploy your troops well in the back of your zone. Dark Eldar can move up to 24" including the assault move in one turn - so keep your troops back! You should be able to get at least one round of shooting off, even if you move second.
Make good use of the terrain when you deploy. Your assault troops should not be visible to him. Put your tanks in front of your non-transported infantry units as a shield - you have plenty of time to move them out of the way if needed.
If possible, assault from the side. Many players have a tendency to move their troops forward in a long line. If your opponent does this, attack him from the sides, thus not granting him too many opportunities to mass assault you. If possible, assault with two units, when you have the chance of wiping one of his units out without getting mass assaulted back. Keep your assault squads to the sides, they should be perfect for flank assaulting.
When it comes to shooting, the priority targets should be in the middle of the board. These are the troops that you don't want to be able to assault you, as they can move to any side of the table. So, shoot them down, in order to limit his tactical possibilities.
Your command squad could be a good tarpit unit. I guess it has Feel No Pain? Lock up single units with it, and use another unit as hammer to smash the locked unit.
Your dreadnought is more intimidating than effective. If he's afraid of it, it can be used as bait. If not, use it lock up one of his weaker assault units in combat - he should have a hard time damaging it.
Last of all, your scouts. Scouts are arguably not the best unit in Space Marine Codex. You get little value for the points invested. Use them as bait. Position them as being able to assault, so he won't assault anything else. If he doesn't care about your scouts, move them towards an objective, or if there are no objectives, consistently put them in his way - there's no need for them shoot if shooting stops them from doing any of the above.
Hopefully, some of the above is useful for you. Hope you will fare better in your next game!