Are mastiffs typically pretty timid despite their size?

Ok I know this question is pretty subjective but I'm curious to learn about the experiences of other english mastiffs owners. I have had my girl since she was 5mths. From the very beginning I have taken her to the dog park twice nearly everyday and to large city events, etc. Immediately the tail goes between the legs and she starts trembling. She's now 15 months and I have to wonder if this is ever going to get any better. I have a 10 y/o shih tzu as well who's a typical shih tzu: chill as can be, and I had hoped that the shih tzu's chill demeanor would be soothing to the mastiff and rub off - not. Just to add, a good friend of mine also has two dogs and one is an english mastiff. Her lab is amazing and loves to run and play at the park but her mastiff will not leave her side, never play with other dogs at the park and once urinated on the kitchen floor when she invited a new friend over. What is up with mastiff? Nothing I have ever read indicates that they're freakshows but I'm beginning to wonder.