The failure to obtain and maintain a solid enough erection for sex is referred to as erectile dysfunction (impotence). Having erection issues now and then isn't really a cause for alarm.
Masturbation will NOT affect length or girth either way, so no worries there.
The only thing I’d say about porn is it can be kind of addictive - if guys get too used to relying on it for masturbation, they may not have that same mental ‘stimulation’ when with a partner.
Masturbation itself will not cause ED issues, but as with porn, if you get too used to a particular type of stimulation (like using a ‘death grip’), again, you prbly will not feel that same stimulation & could lose your erection when with a partner. That would not be true ED , which is a physical inability to get & maintain an erection, that would be more of the ‘psychological’ effect - the ‘plumbing’ (blood flow, etc.) is working just fine, but worry & tension are preventing things from working as they should ...
As GR says, sometimes as guys we focus too much on our ‘performance’ instead of connecting w/ our partner & just enjoying the feeling. I think you’ll be OK if you just relax a bit & not worry or put too much pressure on yourself to ‘perform’.
In the meantime, I would suggest trying masturbation without looking at porn - you can use your imagination or whatever gets you in the mood & just focus on the pleasurable feelings. After all, that’s what most of us did when we first discovered masturbation - I didn’t know anything about porn or sex at that point & didn’t even know what I was doing, other than it just felt good !!