There are a few ways to measure success. First is viral control. And I cannot tell from what you've said if you are getting there or not. One relatively common measure is to ensure the viral load goes below 400 copies in the first 3 months or so. And then by month 6 you should have a viral load <50 copies. This is a bit of a generalization and there are exceptions but it is a good guide. You should at least be steadily dropping towards these if not there in these time frames. And I cannot tell when you started so not sure if you are "on track". And there are reasons to expect your combo will work - as some studies of a combo such as this suggest that many will have viral control. But we also have learned that this combination may not be as potent as others we now have - and some don't get full viral control on this type of combination. If you are "on course" then there is less if any need to alter this for virologic reasons -- but if you are not -- there is a certain urgency to doing so - since the first combo is often an opportunity to establish control without any risk for rebound and HIV resistance that can accompany rebound. And there are some fairly simple ways to modify this combination to make it more potent. For example we have learned that a protease inhibitor that is modified by adding a low dose of ritonavir - so called "boosted" PI - can be more potent than nelfinavir which cannot be "boosted" by ritonavir. Alternatively some might add a third nucleoside if you are dropping slower than expected - one prove choice for you would be to add "viread" or tenofovir although I don't know where you are and if this is available. This has the advantage of allowing us to have you take the ddI with food - since these two have a favorable interaction - if you did this you would only need to take the 250 mg DDI with tenofovir with food to get the correct levels of both drugs.
Now with nelfinavir - the food effect is pretty important so taking it three times daily with something to eat is one key to success. One other option avaiable as well is to change to 5 tabs (1250mg) twice daily - perhaps about 12 hrs apart - again with food. The AZT can also be 300 mg twice daily - and this combo is a bit easier - and likely just as successful. However since the ddI is taken on an empty stomach while the nelfinavir is taken with food - this combo has some burdens which can be minimized by changing some of the choices here as mentioned above. Again, I don't know where you live nor what options you have open to you - but while this can work - it can be a challenge to get this combo correct each day -- and with some minor changing we could make this simpler.
One more issue here is diarrhea or loose stool - a problem seen in those on nelfinavir, and for some ddI adds to this issue. Again there are ways to handle this - including "antidotes" such as calcium carbonate dietary supplements or fiber supplements, or again, alternative meds with less of this toxicity.
As for specialists - there certainly are some in Europe - plenty in fact. I don't think there is any 'web site' to look on to get a listing as there is here in the US - but you can often get referrals by starting with HIV service organizations in the country that is easiest for you to get to. Or just do a search on the Web for some names of people who write about HIV (such as on this site) for various web sites - or who get written about -- as one way to start the search.
Hope this helps and you get the care you need to get this right. There is much progress made in our field and we can do good - and the help of someone who knows HIV is critical since there are great ways to do this well - and things to avoid as well to minimize the damage done when we err...