I just started picking up my acoustic again about a month ago. I have a strong passion for music and want to play reggae in the future. but lately I've been struggling with strumming pattern. like I'll have a pattern down and then switch to another pattern. when switching back to the first pattern I can't play it all of a sudden completely. like the pattern I was playing previously just got erased from my muscle memory. I'm never going to give up but can anyone offer some advice. like sometimes I feel like it's something wrong with my memory rather than me being a beginner at times or it could just go away after time. all comments appreciated. Thank You
Well it sounds like you already know what you want to do and are practicing... Practice more? I know it's like the lamest answer to everything but it works. A few years back one of my teachers made me play Wonderwall and Disarm (Smashing Pumpkins) exactly like the originals, just to get my rhythm tight and my muscles doing what I wanted them too. I find I'm much more versatile now.
So maybe just play the pattern for a few minutes straight a day, concentrate on keeping it tight and consistent. Slow it down if you need, speed it up if you want(sometimes it's easier to do it faster). But try not to think TOO hard about it (I know it sounds like a contradiction but you want to be able to play this automatically). Another thing you can do is play along to a loop recording of yourself so you can get back into the groove when you lose it.
Well that's the best I can think of atm.
I don't think its muscle memory but likely playing one rhythm corrupts the previous one in your mind. The best thing you can do is write down what the strumming pattern is such as

1 2& &4&

Then try

1 3 4&

The numbered beats are downs the ands are ups. All strumming works this way.

Good luck