Thanks for the added information.
You microphone is best suited for studio recording (where there is no amplification in the vicinity), or live sound for very loud sound sources - and significant distance from amplification.
Depending on which polar pattern you choose, it is going to be extremely difficult to avoid direct or reflected sound from the L1 getting back to the microphone. That's the cause of the feedback.
I have had some success with other figure-8 pattern microphones with the L1, but I had to block the back half of the figure 8 with sound shielding.
If you are trying to capture both vocal and guitar, you probably have the microphone several inches away from both sound sources. This works okay for recording, but in a live sound amplification situation (e.g., an L1 in the same room) this is another cause for the feedback.
For live sound reinforcement it's easier to manage one microphone for vocals and a pickup in the guitar. You can use close mic technique with the vocal mic (lips barely brushing the windscreen). This will help you to get the gain-before-feedback you need for live music performance. Using a pickup in the guitar will also give you much greater gain-before-feedback than using a microphone in front of the guitar.
Do you have access to a hand-held vocal microphone like a Shure SM58 or something similar?
Do you have a pickup in your guitar?
Are you performing for an audience?
How many people are there?
What are they doing while you are performing?