I know this is a slow reply, and I hope it's not too late... (I'm in the middle of chemo, so my attention is coming and going.) Seromas are a pain, I had them too. It felt like my boob was trying to grow itself back, just horrible. And the tightness! It felt like I had this huge elastic band sticking around my chest and arm. Lots of weird pain -- at times my arm felt really sunburnt, at other times shooting pains made me jump. And I found the exercises really depressing. The instructional drawings just irritated me, and the exercises themselves just reminded me of how much the whole business hurt. So I stopped doing them. I didn't stop moving my arm, though -- I just did what suited me: really *really* gentle yoga, breathing through the tightness, and stupid dancing. I'd put on Tchaikovsky when no-one was home and pretend to be a ballerina. Much more like playing than exercising!
Anyway I'm ten weeks out now (from the same procedure as you, a full mastectomy and axillary clearance) and my arm movement is nearly back to normal. The area still feels tight and numb, but not as bad as before. At 2 weeks I was exactly where you are, and like you felt that I wasn't recovering well. I still can't do a proper 'downward dog' yoga pose but I'm fine with sleeping and other normal stuff. You'll get there -- find your own way. Hope you're on the mend now.
Go well,
Heather