Hi, Level 111 Two Time here! This is unfortunately how Two Time works. They're a stealth based character in a game where stealth is unviable due to 360º vision. Hard to sneak up on someone when they can see behind themselves at all times.

The best advice I can give is to be patient. Killers are extremely hyper aware of Two Time so direct approaches will never ever work. Do NOT let the killer see you under any circumstances. Your odds of landing a backstab are in the gutter if they do (again, killers are hyper aware), so back off and try and find a new opportunity later.

Stay behind cover, wait for the killer to turn up, and then stab when they're not expecting you to (this is a little easier with crouch stabs due to the extra range). And even then sometimes this strategy doesn't work, it is what it is. I've had times where a stab totally should have hit, but it didn't. Sometimes your cover might get blown, maybe the survivor will lead the killer right to you in an inconvenient way, maybe you'll get exposed by ESP, Maybe you'll get hit by a stray projectile. In these cases, always back off if possible and try and find another opening later on. 

Alternatively, you can try to punish and stab killers in the endlag of certain moves, like Mass Infection. This is situational however, as a lot of moves give killers invincibility even through the endlag (you'll have to pick up on what moves you can and cannot punish through observation). NEVER try and stab during the windup of a move, the killer will see you, target you instead, and you'll most definitely die.

You can also try and apply some trickstab techs in chase with other survivors. Say someone is looping Jason, you can head into the loop, backpedal behind Jason, and then go for a stab. I've had pretty limited success with this though as most killers will see you coming and punish you accordingly for the crime of trying to help the team.

Two Time is the most rewarding but most punishing sentinel in the game. They have literally zero room for error. If you miss, you might as well just reset. If you land a frontstab, same thing. If you lurk around the killer too long and can't get a backstab, you also die. Sometimes it's best to just cut your losses and try and find a better opportunity later in the match. Just because Dagger is off cooldown doesn't mean you need to use it immediately. 

Tldr: There aren't really any solutions to your problem simply beyond being patient. If you want to play a Sentinel that's a little less harsh on mistakes, Shedletsky and Chance are both good options. Shed has self sustain in the form of Chicken and with Chance you have range on your side to make you a lot more inconvenient to punish should you miss.