Well, two occasional problems really...
Ive numbered the topics at wich I would like to get a reply the most, hopefully I can get a little bit of help!

Generaly, I have no problems with my sex life.

I masterbate every couple of days (When my GF is not available), and I can hold an erection for hours on end if I feel like it. (Generaly dont...)
Also, after ejaculating, I can regain a second erection in 15-30 minutes if desired.

However, when I am with my girlfriend, I do run into problems.
She is very attractive, and I love her very much, and I do have a strong desire (She thinks its too strong), so I doubt the problem is there.
(1)I do however feel a strong need to satisfy (And usually do with multiple orgasms, but it is usually on my mind during sex, Stress problematic?
(I do try not to think about it and just enjoy myself, but dont seem to be able to, and do feel great joy at having her get a good orgasm.)

(2)I sometimes lose my erection during intercourse for no apparant reason.
Everything goes great, and then I can just feel it going away.
This usually occurs after several minutes of intercourse, and when I start feeling the loss, I can usually push a bit more and have her achieve orgasm even if I dont.
(3)Also, I do generaly have problems in retaining my erection if there isint constant stimulation to the penis (Just a few seconds...) like when reaching over to get a condom and so on.

My girlfriend has the most problems with this, as she feels its her fault. It makes her feel inadequate in the sense that she fails at pleasing me.
We have the most problems with sexual position in wich I am not in control.

(4)Is it a problem with her keeping a good stimulation on my penis with her vagina or is it a problem on my part?

This problem is a rather weird one, and quite infrequent (5-10%) but still a concern... I sometimes have issues with achieving an orgasm. It seems that in these cases, we have sex for 30 mins and I am no closer to reaching climax, and honestly, by then I am completly worn out (Im not in great shape) and often just give up. I then retain my erection for 5 mins more and then it simply goes limp normaly as if I ignored it the whole time.

I dont really mind it that much as I find as much pleasure in giving her an orgasm as having one of my own, but again, she feels that it is her fault that I am not getting as much from it as she does.

(5) Any solutions to this? And is it a big problem or not?

We do generaly use a sexual position that enables me more feeling and deeper penetration to ensure that I achieve orgasm, wich sadly dosent allow much variety to our sex life. Missionary with her legs up usually works best for me, so the many variations of that position is what we usually do.

We also always use a condom, wich I do believe is part of my problem as it removes some of my sensitivity, and my girlfriend is not using any contraceptive methods...
(6) Would it be a good idea to try and lose the condom and use the pill. (We are both faithfull to each other and have no STDs)
And if so, how should I aproach her about it? She is rather shy about her sexuality, and never actually looked into it at all.

Awaiting any reply impaciently, and thanks in advance for any advice!