Jack,

Unfortunately, what you are going through is an internal struggle for anyone who is good at heart and overly nice to others. I have fought a similar battle for many years–sometimes, the best way is to simply observe others’ behavior at work and mimic it. Adjust to survive.
Otherwise, people will grab any chance to take advantage of you! And not only that–they can measure you up in a matter of seconds. And if the work culture favors confident and arrogant behavior, then you will definitely be an outsider. Many religions teach compassion and selflessness but what the other bloggers have advised you is correct–you can’t love other people if you don’t love yourself first. You can’t command respect from others if you don’t respect yourself first. You can still be a nice person and offer to help when needed, but there is a difference between being polite and helpful, and allowing people to take advantage of you. If you don’t stand up for yourself, no one else will. Compassion is a very precious gift in our cruel world–you shouldn’t give it away to just everyone. Most people don’t deserve it.
Another piece of advice–don’t be afraid to admit in front of others if you don’t know something–for instance, that you are still new at your job and learning. It is a better option that than coming across as generally insecure. Once you explain the reason to others, they even may try to help you.

Best of luck! Building self-esteem takes time but all you need are just small steps every day. If you believe you can make it, you will! Buddhism also teaches us of patience and persistence–you can use these in your confidence-boosting journey.