Two words; Stop loss

Once you get down a few buy ins it's very tough to come back. Most of the time you never get the chance because if you double up once or twice they leave. Even though you got it back this time, that isn't the norm in this situation. Confidence is huge in HU play and once you feel like you're getting crushed and nothing you do matters you're pretty much done. The other player is going to play much better because he has the momentum and you will play much worse.

I sometimes will stick it out, but only after I take a few minute break to make sure I'm ok mentally to continue. If he leaves when I ask for a 5 minute break so be it.

I think taking a break is not a solution to the problem. It works well as a palliative like painkillers, but the backbone of "not-optimal mindset" is not changed. 

taking a break is not the solution to the main problem. but having a conservative BR management when you are playing your D game is. 

quit, start taking notes on your mental game and get back when you feel comfortable. 

Realize that HU PLO is a game with a lot of variance. Sounds like you may have some BR management problems if a few buy-ins gets you into super-tilt mode causing you to lose 10+ buy ins.

LAGs in PLO are occasionally going to run very well for a seemingly long period of time.  For one they are obviously doing one thing right in poker which is aggression.  Any time the base of your play is aggression you have a chance to dominate a player or field for a period of time.  When this player hits a patch where, due to dealt hands or board runouts (or both), he's consistently in the top of his range it is very likely that you are going to get run over.  If you can focus on your A game and not tilt playing villain will still +EV as we can not predict when these random sequences will begin or end.  Every hand is an independent event in one sense.  But if you feel that the losses are causing you to play a less efficient game (ie tilt of some form) you should probably quit.  Not only because playing less than your A-game may negate your edge, but also because you are already losing the information war.  Anytime a player is consistently running you over he's gaining information about your ranges and how you play boards and what you do with weak to medium holdings.  But you are gaining little info about his game because he is doing what he does (betting and raising) and you have no idea where he is in his range.  So you are already playing at an information deficit.  If you can't overcome this deficit with better play then you should quit and wait for a better spot.  Remember, villain's prior success in combination with a period of fortunate variance is only going to improve his play and focus and further embolden his aggression.  Few poker players play worse while they are winning.

I've played many many times where I look back and realize that the LAG I wouldn't quit was just in a much better place to win than me.  It may be the variance that I had experienced during our match, or some tilt leaking into my game.  Or simply that the villain's game matched up particularly well to exploit what ever leaks I had in my game at the time.  This is the value of a stop loss.  1) it can prevent tilt.  2) it can stop us from proceeding in a situation where we are at an informational deficit due to variance.  3) it can stop us from continuing when we face an opponent who may (intentionally or not) be exploiting our leaks.

Just read mental game of poker and all will be fixed

focus on learning and not winning, and it will all sort itself out.  Players mistakenly do the opposite, and it's disastrous for the bankroll AND mental health. Learning is the fun of poker, winning is just a meh bonus.  If you're thinking about it another way, you're doing it wrong :)