Basically I've been with my girlfriend for about 6 years, the sex has gotten worse as time goes on. Literally it tends to be just missionary a couple of times a week. I have tried talking about it but it ends up in arguments. I love blowjobs but can't remember the last time she gave me one.
Welcome to 'the club'. The reasons you quote are probably the largest numerically why guys pay for sex. It's satisfying, fulfilling and no strings.
Dating sites are a risk as you've discovered, but speaking personally, you need to ask why alarm bells are not ringing in your GFs head. Clearly she has got the message you are unhappy in the sex dept, but from what you say, she's not prepared to do anything, and while I am not a qualified marriage guidance councillor, that's not the basis for a good foundation for marriage.
And neither is illict sex, paid for or not. Yes, it's bloody fantastic at the time, but if paying, you will find it can be addictive, and then questions will be asked where they money is going. There's a risk of being caught out, an outside risk of an STI, etc, emotional attachment syndrome with the pro$$ie, so you really need your head screwed on and to be mentally strong. Many guys have been caught becasue of simple lapses,(like spunk stains in pants, etc) so if you have a PAYG phone, wipe it after every punt and have a valid reason for having one, should it ever get found. I knew of a punter who hid his phone in the toolbox in the garage, and all was well until his wife wanted a screwdriver for something, found the phone, no pin code and lurid text messages for her to see. His bollocks were cut off first, then she divorced him. My punting phone doubles for things I sell on Gumtree from time to time, so has validity, but is still wiped after evey punt, regardless of how nice/sexy/horny the messages are. And for God's sake, dont use it to take pics of a WG giving you a BJ
While this ain't a lecture, maybe because she wont give BJs for reasons best known to her, things have run their course and you should find a GF who will give you what you want in bed. Usually, the sex tails off after marriage, not before, which is why most of us are on this forum.