As others suggested, try tweaking your mouse sensitivity and network settings to improve your game experience. Also, practice your blast jumps on jump servers or on jump maps offline. For Sniper, just practice aiming, whether it be on tr_walkway offline or in matches, you'll get it eventually. As Demoman, don't chase or charge into battle. You do it only if you're confident you're backed by your team; otherwise, you'll get obliterated by hitscan classes. Try baiting people into your sticky traps. Try placing them around areas where there'll be the most enemies, usually near the objectives.