I'm 18 months into my lab based PhD, and all i seem to do is mess up experiments and get stuff wrong. I've not done appropriate controls for some experiments and so had to go back and repeat them. I've also used the wrong concentrations of drugs and had to repeat these too. It wouldn't be so bad but I use primary cells from patients so patients have to keep coming back in so I can do this. Recently I've done a massive cock up, essentially the experiment I wasn't doing wasn't properly optimised and so now the data I've got is obviously questionable.
I just feel like an absolute idiot. I can't seem to get anything right, and these mistakes are picked up by my supervisor, it's not as if I even spot them before it's too late. I really enjoy my PhD (some of the time) and I desperately don't want to quit, but I'm starting to think that my position in the lab is becoming untenable and that maybe I should consider leaving as this just isn't for me. The experiments I'm doing are fairly basic, but if I can't even get these right what hope for the future is there? I really don't know what to do. I spend half my life crying at the moment as I just feel like such a failure. I thought after 18 months I'd have at least learned something, but it just seems that I haven't.
Any advice?

Don't worry too much, everyone makes mistakes! :-) It's a steep learning curve, and we're not expected to know everything or get everything right.

Do you sit down and plan your experiments through before you start them? Is there any way that you could go through them with someone before you start to make sure that it's all planned out ok? :-)

Hi Starshine, it's absolutely normal to make mistakes at first. There are so many things you need to learn and master about designing and carrying out experiments correctly. In fact that's why you do a PhD, to learn skills like experimental design in order to become a fully trained researcher!

Does your uni/dept offer courses in experimental designs? if not, ask your sup/colleague for a good book, and read it cover to cover. It'll make you a more confident designer.

I am very proud of my precious mistakes! Some of them are very unique, some others are really common. I learn from my mistakes, I remember each of them for ever. Never make the same mistake twice.
I make jokes about them and inform people who are just starting " Don't do that, it doesn't work". For example, when I was 5 I put my arm floats on my legs and almost drowned. (up) I ve done numerous mistakes in the construction site (each mistake costs a lot of money) and an awful lot of mistakes as a researcher.
I am glad I am not doctor or an air traffic controller: each mistake could mean lives. Therefore, complecated tasks are normally performed by a team.
Try to plan your experiments ahead in every detail and ask someone's more experienced opinion before conducting them.