I am not an expert, but this is what I have found (struggling) over the past few years.
I typically have the same issue every year. It doesn't seem to happen as fast though. I typically have to do this about 2-3 times a day at the beginning. What I have learned is that:
1. You have to regenerate the a lot when the water is dirty because the DE filters do such a great job picking up junk it clogs the fingers. Make sure to follow the instructions correctly. If you have to fill up your pool with water at the opening season, make sure to add some extra water so you can backwash many times if needed. Especially if the pool is green.
2. You may have to play around with the amount of DE you put back into the filter after regeneration. Typically it's 2/3 the regular amount, but some say more/some say less. I found a little less works for me.
3. If the water was green, make sure it does not go back into the pool after regenerating. Check the return in the pool when you turn the filter back on. If you see a bunch of green come out the return, you may need to take the filter apart and clean the fingers thoroughly.
4. Once the water is clear, your pressure should stabilize to probably a little bit more than the running pressure you get right after you turn it on. If you are still gaining pressure and losing vacuum, you may not have the correct amount of DE in the filter or you may have added it too fast and it is clumped up in the fingers clogging the path of water.