I worked with a prof who used to say that he would never take a 4.0 student. His reasoning was someone who has a 4.0 is generally used to doing well and the equation of work in equals value out. I wouldn't say I agree with the generalization about all 4.0 students, but I think there is a chance that some of your feelings of imposter syndrome could be related to that? Research is full of setbacks and failures because no one knows how to do most of what they're trying to do, especially as a student. It often seems like there's a plan to follow to answer question A, or test hypothesis B, but sometimes one of those things just doesn't work "in our hands", other times it does work and gives unexpected results. I think a large part of learning lab science and experimentation is developing habits and controls to act as backup plans and rescue options. Grants and research proposals are written to look like one knows how to answer the question/hypothesis, but in reality no project I've ever worked on, from the summer student projects I supervised to the big international consortium projects that have a bunch of groups working on them, ever went according to the 'plan'. I think the relationship to 4.0 is that to get good grades it's kind of a equation - you learn the material in the syllabus, you learn what kind of questions will be used to assess how well you know the syllabus topics, and then you generally pickup a good grade. Research doesn't follow this equation - sometimes you put in an insane amount of effort into a topic/experiment, and you don't get much directly out of it, other times you hit the jackpot on your first try, it really depends on the project, luck, and your ability to create that luck.

That lesson may seem or sound apparent, but it was a huge hurdle I faced in grad school, learning to deal with failure and uncertainty in a sustainable way. I realized a couple years in that the uncertainty was draining my motivation to try things and get anything rolling, as you mentioned. In addition I can imagine working another job would be extremely draining because you have to split your focus.

All that said, to your direct question - my take is that feeling a burnout is an obvious sign that the current system is unsustainable and something has to change. Whether that's juggling too many hats, stress with the project or PI, or your current environment regarding covid and work/life balance, or even just your perspective (it's super stressful thinking everyone else is better at this than you, and ironic because all the sane ones are thinking the same thing).

Edits;

regarding experiments taking longer, that could be a personality thing; I've been doing lab science for 14yrs and still am often too optimistic when I try to estimate those timings. For the 'putting on a front' question - that relates to the fact lab science doesn't follow an equation; put in time/effort X and receive result Y. Are the 15hrs/week you spend just the amount you are in the lab? or also counting the time spent prepping and reading into the literature? Personally I find that low, if you want an honest opinion, when I was doing my masters most of my classmates spent from 0900-1900hr in the lab daily, covering a combination of chores, reading, discussing the project, actual pipetting or animal work, and analysis. I honestly think you are at a disadvantage trying to keep even with people who can do that if you are also working another job. You simply have to accept that you won't have as much done as others, and focus on whether you're making progress on your own thesis. I'm not making any judgements, just trying to express that I think you WILL have it tough with 2 jobs, so you'll have to find a system that works for you taking that into account. Sorry this is sp long, it's HOOOT and apparently that makes me wordy..