How do I design a system with tough constraints?

You can design most systems now, but the complex, tightly-constrained problems still defeat you, the ones where every requirement fights every other and there seems to be no room to move. That difficulty is not your limit. Heavily constrained design is genuinely hard, and struggling with it is how you learn to do it. When constraints all pull against each other, the trick is not to satisfy them all at once. It is to find which ones are truly fixed and which have give, then design toward the hard ones first and trade off the soft ones. A good design is rarely perfect on every axis. It is the best honest balance among things that cannot all win. With the next hard one, list every constraint and mark each as firm or flexible. Build your first design around the firm ones, then negotiate the rest. When two truly cannot both be met, name that plainly, to yourself and to whoever owns the requirements. Great constrained design is not magic. It is clear thinking about tradeoffs, done patiently, one constraint at a time.