What legal steps can I take to stop a neighbor from shinning very bright floodli lights onto my front yard and my windows?

I am not licensed to practice in WA, nor do I seek to establish an attorney-client relationship. The following is just informational and based on general legal concepts.

While it would be helpful to have more facts, especially about the nature of the lights and how they are shining on your property, what you are describing would probably fall under the law of nuisance. Under Washington law:

RCW 7.48.010 - Actionable nuisance defined.

The obstruction of any highway or the closing of the channel of any stream used for boating or rafting logs, lumber or timber, or whatever is injurious to health or indecent or offensive to the senses, or an obstruction to the free use of property, so as to essentially interfere with the comfortable enjoyment of the life and property, is a nuisance and the subject of an action for damages and other and further relief.

RCW 7.48.180 - Abatement does not preclude action for damages.

The abatement of a nuisance does not prejudice the right of any person to recover damages for its past existence.

I would contact an attorney. They can help get this stopped and perhaps recover damages for the 20 months you suffered with this.