While there’s not one magic bullet that will work for everyone all the time, describable in under 1000 words, I can make a suggestion based on the information you’ve provided.
Substitution exercises:
Take one of the sentences that you’ve learned, and practice switching out various elements, one at a time. For example
I like French fries with mayonnaise.
I like French fries with ketchup.
I like Hamburgers with ketchup and mustard.
My friend likes Hamburgers with ketchup and mustard.
My other friend doesn’t like Hamburgers with ketchup and mustard.
Note: I recommend doing this exercise out loud, *not* in writing. The idea is for you to increase the speed at which you can form sentences while speaking in a conversation. Practice slowly for accuracy, and slowly build up speed as you feel more and more comfortable. Have fun!
One of the nice things about English is it is pretty much universally spoken at a much slower speed than Spanish is, so while you might feel like you are speaking slowly, it's probably not significantly slower than an average native speaker. The reason for this is because English has more information per syllable than Spanish does, so English is able to convey the same amount of information with fewer words in the same amount of time which in turn makes it slower. For some concrete statistics about this, English has a high information density
of .91 and is spoken at an average rate of 6.19 syllables per second.
Spanish, on the other hand, has a low-density of .63, and speeds
along with an average velocity of 7.82 syllable per second. While most native Spanish speakers don't even notice it, this is a primary reason why native English speakers generally have a difficult time understanding spoken Spanish. For them, it is extremely fast -- slightly over 25% faster!
Anyways, to specifically answer you question, I have a couple of suggestions:
1) Practice tongue twisters. These are phrases that specifically target increasing your rate of speech while still retaining clear enunciation.
2) Read out loud whenever you read anything and practice reading at different speeds. For example, read a sentence out loud at a normal speed and then read it again a little faster. Repeat this while increasing the speed several times until you reach your desired goal speed.
