Seeing a video of you playing/the point where you feel your technique struggles would be great, to give the best advice. As there are so many variables that could be holding you back.
Everyone's advice is spot on. Stay slow & relaxed until you're comfortable. From experience as a Guitarist that's been in an out of the obsession of picking technique my whole life - Your body will 100% tell you if you're doing too much too fast - and you'll save so much time if you listen to it. We all have that inner speed greed inside us that makes us want to increase the tempo when we decide we're ready - way too early. It's something I still struggle with all the time
The main points that have always helped me with Sweep/Economy picking is the realisation that the speed comes less from the speed and more from the well trained synchronisation of left and right hand, and that can only be trained in at a slow enough speed that you can make the decision to match the right movements up. This is why it's a slow process at first. I'd keep at it and use a metronome constantly to gauge your progress. The main thing is feeling In control of what you're playing at a slow tempo. You'll know that feeling of holding on for dear life with a fast technique - whenever you feel out of control, immediately slow down and it'll do wonders for your playing
Rememberr you're training two sets of small muscle groups to work together in tandem, it's going to take a while.
The only other tip I can give is to make sure there's no undue tension in your playing, I.e squeezing your pick too much that it fights the strings, tensing your hand/forearm when the speed increases. It should all feel as natural as possible, but still controlled