Viola palustris or marsh violet is a common plant from wet soils with low nutrients. You can find them in wet heaths, fens, marshes, bogs etc.
They have a unique flower colour as they are less blue than the other true violets and instead are lilac to even reddish lilac. Always with darker veins.
Early in the season when the plant flowers the leaves are small but these grow a lot during the summer making them look like entirely different plants