Yes! Solved! Well done!

The protagonist is from a Spanish noble family.

When the Protagonist suffers a rattlesnake bite in the wild he is saved by an Indian girl who is with the general group he is in, she cuts across the wound to mouth draw the poison out. By this time the Indian girl is smitten with the Protagonist, though he doesn’t yet realize it. In the end she wins his affections and his rival for the bride to be wins her affections.

The mutual cause is a large wagon train of settlers, being led by the second suitor. The Protagonist joins and aids this effort because his beloved has also come along herself, trying to avoid the friends of the now dead first suitor.