Hi mik5,
I am also a BT, and used to daven mostly in English. I transitioned gradually to davening in Hebrew by:
1.) Learning to say the ending sentence of each paragraph/psalm, e.g., where the diamond-type mark in the Artscroll is to the end, in Hebrew. Since that diamond mark is where the chazzan typically begins reciting aloud, learning those sentences first really helped me to figure out where the tzibbur was in davening, and that made me feel a lot more comfortable in shul. I started with the the ending sentence of each bracha in Shemoneh Esrei, then the final sentences of the brachos of kriyas shema, and then did final sentences in pesukei d’zimra (the psalms/verses recited before shema during shacharis). Pretty much all the rest I did in English (unless I happened to know some of the Hebrew from my non-Orthodox Hebrew school upbringing).
2.) I gradually added the first sentence of each paragraph in Hebrew, slowly so that my davening didn’t become excessively long. Once one Hebrew sentence became quick and automatic, I’d begin working on the next.
3.) Once everything was “up to speed”, I’d add the second sentence of a paragraph, and the second to last sentence, etc. until eventually I was doing almost everything in Hebrew.
As I did this, my reading skills dramatically improved, so my rate of progress increased as I continued adding more Hebrew to my davening.
Regarding understanding what you are saying: I found that I learned a LOT of Hebrew words during the transition from reading in English to Hebrew. When I daven Shemoneh Esrei, for example, I understand what I am saying in Hebrew. The area that is still a challenge for me to understand each word is pesukei d’zimra, but I understand parts and keep the theme of each psalm in mind as I recite it. Sometimes I will also take an English siddur and review the psalm in English, so that my understanding is fresh.
I hope all of this is helpful to you!