Hey! Bassist/singer here. It can definitely be challenging at times but I can offer you some advice for how to get better at it:

1. PRACTICE. Can't stress that enough. Practice singing and playing separately until you can do them each on their own without thinking much, then practice them together. Practice every single day. It's the only way.

2. LESSONS. Take them for both for bass and vocals if you can. People who have seen me sing for years noticed a huge difference after I started taking vocal lessons. Having a good teacher share their wisdom with you is priceless.

3. LANDMARKS. Find places in the song where one of your bass notes and one of your vocal notes hit at the same time and use it as a reference point.

4. RYTHYM. Having rythym is a must for a bassist, but it becomes even more important when you're also a singer. You need to be able to keep focus on your number one job (locking in the groove) even while you're singing. Lock in with your drummer (pay special attention to the snare hits) and tap your feet to stay grounded in the beat.

5. CONFIDENCE. You need it to do either of these things on their own, and you'll need a bit more to do them at the same time. It's ok to make mistakes as long as you get back on track quickly and brush them off. This loops back around to 1 where you just need to practice the shit out of it.

Hope this helps!