While I don't know of a good Bolster guide (I think there used to be one stickied, but that may just be my imagination), I'll give you a couple tips to make your experience a bit more pleasurable.

 

Regarding Operatives, try not to make yourself an "attractive target". This will be kinda difficult, because you are a lowbie Sentinel, and that is already a pretty attractive target for most classes (especially those that know you will be dominating them as soon as you "come of age").

 

If you know there are stealthers about (or have already died to one recently), try to stay w/ the group. Much like lions, Operatives love to hang out in stealth and pick off the weak antelope. DON'T BE THAT ANTELOPE. Stay w/ your herd (pack? flock?) if you can, so not only will you have support if they do pop out on you, but more people being around will give them less opportunities to get near you w/out being spotted.

 

Low health... This is like a bright, shining beacon for most players to target and kill you. This is actually directly tied to why most people absolutely HATE Sentinels, because Undying Rage (Guarded by the Force for you) turns that last sliver of your health into 5 seconds of Super Mario Star-mode. People hate being robbed of that killing blow, especially if their assumed killing blow turns around and KILLS THEM instead. ...but I digress, if you're low on health, get to the middle of a group pronto or fully expect to get picked off.

 

Basically any lone straggler is fair game for a DPS operative, and if you are low enough health to be taken out in a single stunlock (about 60% of your hp), then you can almost guarantee one will be coming for you if they are around. Operatives are fairly squishy outside of stealth, and if they can kill you and drop combat to restealth, that's really a best case scenario for them.

Super Mario Star Mode, best analogy ever of sent/mara defensive cooldown.

And yeah, at level 14 sent/maras do not have enough of their defensive stuff to deal with operatives, nor their offensive punch. At level 50, fighting sent/maras on my operative is always a struggle of me jousting and line of sight healing, because if I stay in melee more then 5 seconds, I lose 75% of my health. I have to constantly be on my toes as an operative at level 50.