1998 Nissan Frontier A/C only works on highest setting

My A/C has been acting up like this for almost a year already. I had no intentions of fixing but the 104 degree weather in TX is starting to become unbearable.

I noticed too, that when I have it on the highest setting for awhile, maybe like 20 minutes, it stops blowing out cool air.

The only work I've done to it so far was replace the A/C relay with no luck on fixing the problem.

Any ideas why it's not working on the other settings?

Thanks for your help.