Thats not bad at all for an opening batsman. A slight back and across initial movement has been used by many openers as the launch for the next movement. Gavaskar is a prime example.
Now coming to swing. Being a backfoot player is not bad at all to counter swing (although most batsmen being predominantly front foot players flounder and prefer the apparent safety of the front foot). The problems could be in many areas. Lets try and see if we can get to this clear in stages.
Playing off the backfoot
Backward movement
We are starting with your statement that you have no problem picking the length. So I assume when you move back it will be to a short pitched delivery and not one of fuller length.
When you move back do you move right back or just across when you say you move back? Just moving across but not back is fraught with danger. You need to move back towards the off stump (actual movement will depend upon line of delivery). The further back you move, the more time you get to see the movement off the pitch. Moving across but not back towards the stumps will only mean you are going to be leg before instead of clean bowled had you not moved at all.
So number one - Move backwards too and not just sideways.
Sideways movement
There are bowlers whose swing starts early in flight and others who will swing late. (We are here refering to movement in the air or swerve and not off the wicket (seam or cut).
Generally, unless you are an opener facing the first ball from a bowler you have never played before, you would have an idea as to what he bowls. In junior cricket it is not always the case and openers facing the first delivery can get into trouble with a swinging delivery they did not expect. But we are here talking of a persistent problem you have and will assume that you know what the bowler generally bowls.
I am also assuming that technically there are no serious flaws and that you are by and large side on when facing the bowler in your stance and reasonably side on as you move back and across.
So if you are facing a bowler who swings away (out) from you, you have to move more than if he was swinging in. Mt coach gave a very simple thumb rule which I followed through out my playing years and it worked pretty well though later it becomes a habit or style.
He said, for an out swing bowler, move so much that your left pad (we are throughout going to assume you are a right hander) is in line with the point where the ball hits the pitch. This will mean that if after pitching the ball continues in the direction of the swerve, it will end up in line with your right pad and where your bat should be coming down for a straight batted defence or stroke.
Remember, all technique will be kept simple for normal movement. If there is exaggerated movement the best of techniques will be beaten but thats what superb deliveries are supposed to do.
If the ball straightens after delivery, it will still come towards your left pad and you are still going to be in a reasonable position to play it with a straight bat down the line or towards the onside.
Exactly the reverse is the case for an in swing bowler.
You move so that the right leg is in line with the point of pitching so that if it continues moving you can play it off the left pad and if it straightens you can play it off the right pad.
If you are okay so far we will continue.
If you have any questions.. Shoot.