I know a few guys that had this problem and for ALL of them the doctor said that their was nothing physically wrong with them, it was mainly just the anxiety. The fact that you catch wood when not with a girl and totally relaxed proves that you have no physical issues.
Some of the guys were perscribed these "anxiety" pills, like xanax, or some ish like that, and for a lot of them that will do the trick. The guys that needed more help were perscribed different doses of cialis. For some a small 5 mg will be just fine, others took up to 20 mg. But obviously try first with just the anxiety pills, that could solve all your problems. In other words, go see a specialist, that's what they're there for.
Also, if at all possible, stop watching porn alltogether. This puts unrealistic expectations of sex on a lot of guys and can make you nervous or give you anxiety. Seriously, just STOP, alltogether.
Also, focus on pleasing her. You can still please her even if you don't get wood right away. Use foreplay, learn how to treat her body with your hands and mouth, learn different ways of playing with her and fingering her. Once you become good at this, if you are ever struggling just start playing with her, and once she start moaning from pleasure, there is a VERY GOOD chance you'll catch wood, and once you got good wood going, just hop on top of her and slide in.. slowly at first obviously... lol... learn to also give her orgasams not only with your wood, but also your mouth and your hands, all of these are gonna help you also get more turned on and get wood..
ALSO, have her suck you off, this is usually a surefire way of getting wood. If this is a first time or somethign, then obviously play with her first, try to make her orgasm first with your mouth/fingers, whatever you can use lol... and once you do, if you STILL don't have wood, just tell her "give it a little kiss".. or however you wanna tell her up to you, just throwing ideas out there.. most of the time after "give it a little kiss" (you can also use this if you already have wood and want a BJ, obviously lol, or if you have semi-errect wood, but would like to solidify it),, MOST of the time they will just go for it right away as they get what you want.. the far few and between that actually do just give it a little peck, just come back with "hmm, I know you can do better than that" (confidentely and playfully), and almost always they will giggle or smile and get to work..
ALSO, if you're fingering her, feel free to use your other hand and jerk your self until you get wood, and then proceed. Never be afraid or shy to jerk your self in front of her, it's all gravy! You can also easily jerk your self while your'e going down on her..
ALSO, being in great shape helps, and NOT using steroids helps lOL.. (hey, some guys juice up) So be in shape, go to gym, and you also want to be somewhat on a lower fat percentage scale, so for you with these issues you may want to do more cardio than an average guy..
If you learn how to please her without your pecker, you will become much better also with the wood and also much more likely to get wood since you can spend significant time PLEASING them in many different ways.. usually heavy moaning and screaming will give any guy wood, so learn how to do these things to her and in those instances where you don't have instant wood, do a lot of this (you shoudl most of the time use a lot of foreplay anyway, dont' be in such a hurry to have sex)... and even if you DON'T end up getting wood, it is OK, do not worry about it so much man, that's why your'e not gettin it in the first place, as you said, you're too worried about it...
But definitely also don't be afraid to see a doctor (specialist) and get your self checked out. It could be that your testesterone levels are low, and a doc will hook you up to get this back to normal levels..
Ohhh, almost forgot, one of my buddies that is having issues with his wife (possible divorce) one of the problems was that he was not getting turned on anymore.. and he did not want ANY help... (I told him all this that I told you, but he said "no, that's bullsh*t, I shoudnt' have to go to a doctor for this, it should be natural, bla bla bla, if she can't get me hard anymore, than it is what it is, bla bla) in other words, he was too damn stubborn... he felt like getting help was beneath him.. I told him " you know what, it is YOUR JOB to please your wife man.. you signed up for this, she is WANTING you, and WANTING your sex, and you are not "cuming" threw for her" and after she pretty much threatened to leave (obviously this cause other issues in marriage, they started pis*ing each other off more, getting mad over stupid ish, but you know what, MOST of this is solved by good sex! Good sex actually PREVENTS MOST problems in a relationship, more guys need to become aware of that! So think of that next time you only wanna "get yours" and stop) Anyway, back to the subject, after she threatened to leave, he went and got him self checked out, and GUESS WHAT?? The doctor found out that he had abnormally low test levels for whatever reason, so he gave him some sh*t for that... I guess he's doing better now, I'm not getting too many updates since I was being "too annoying, and he didn't need help" (some guys are just too damn proud for their own good, and would rather lose then get "help"), and I happen to care a lot about both of them and wanted to get it through his thick skull!!
So yeah, your test levels also might be low, so definitely get your self checked out...
