OK my problem is I'm 30 years of age and I have had this problem for quite some time but I'll be having sex and it'll be going really good but then I go limp or sometimes I go too quick but when I masturbate I can go for a really long time (to where my arm hurts from the constant movement) and I want to know why is that because my fiancé is 20 and she was saying that she wants to have sex with others now... please help

You cannot maintain an erection OR when you do you ejaculate too quickly and because of these problems your fiancé wants to have sex with other people. Am I correct?

And you want to know how to sustain the erection longer and/or not ejaculate so quickly?

Yeah basically while having sex I can't maintain it and when I can I go too quick like 5 minutes to 10 minutes but masterbating I can maintain longer

And your concern is that she is going to have sex with someone else because she's unhappy with your performance?

Yeah basically like I'm afraid that she isn't happy in me trying to fulfill her sexual needs because of this problem and all I want for her is to be happy and so I want to know what are the possible problems and what I can do to fix them if possible

An exam and a chat with your doctor would be helpful, to rule out any medical issues.
On the drive there I would wonder why my girl would not have already suggested it, but rather threaten sex with other men as her solution to your problem.
No doubt the pressure from that hasn't helped at all.
I am not trying to be rude just practical. IF she has suggested this I apologize now.
But I'm not gathering that it has been brought up by either of you.

I think her comment that the fiancé wants to have sex with others "now" is making the situation worse by increasing the pressure to perform.

And, yes, OP should consult with a Physician. He should also take a close look at the relationship with his girlfriend.

Actually I have made mention to talk to a doctor in getting a physical and finding anything out but she had made mention of this a couple of days ago but I don't know

Yes many men find that they have some sex problems, from the excitement of a young girl ( your hand or old news) but also you can control everything with your hand, the pressure, the speed so you know how to make it last.

But there are many reasons, both mental and physcial why it does not last, you won't know without a doctors visit.

As for as the girl saying if he does not get better she wants sex with someone else, dump her. She does not care for you, and is not concerned.

Now of course most women do not climax from intercourse, so I assume you are doing oral or other foreplay to have her climax before you start regular intercourse ?

I usually finger her and do oral before hand yes

1. Masturbate less (so you are energetic when it counts)
2. Relax and enjoy the moment (don't get caught up thinking about your previous unsatisfactory performances, a lot of impotency due to mental/emotional distractions)
3. Use a condom (decrease sensation for longer stamina)
4. Be confidence (you have scored a hot young one, what's to doubt about yourself?)
5. Tell her threats are not going to help you perform better (if anything, more distraction for you)
6. Kegel exercises

Sounds good but yeah about using a condom a lot of time out of not I usually don't last with a condom due to the lack of sensitivity yeah I know it sounds weird and all but yeah and also when I'm sick I can't get any kind of feeling whatsoever and that's masterbating and otherwise

I would think that not masturbating would cause him to finish too quickly - ?

Yeah that's what I've heard