I am by no means and expert on fasting, but perhaps you could cut down on the amount of food you take in during fasting periods? For many, just the vegetarian/vegan diet itself is challenge enough. If you've managed to make that a part of your everyday life, then during fast times, you could focus on eating less and, most importantly, fasting from sin. Most of all, talk to your priest about it. Holiness and theosis are life-long processes and things such as fasting are means to those ends, not the ends themselves. He'll know what to do you for your particular situation.