Hello!
First of all I think it can be quite common to feel this way, your partner is getting out, having time to do something he wants to do, without the responsibilities of children etc - of course you're going to feel somewhat negative!
You need to accept that this emotion is normal, human and then you can tackle the situation in a healthy way, for him as well as yourself
So - are there any things you enjoy doing? And it doesn't have to involve going out, is there anything that you can think of that you'd love to do?
I'd suggest that you work around each other, and maybe come up with a plan/schedule, in which you get some time for yourself and your own interests.
My partner goes out once a week (some weeks more) and some nights has time on his Xbox, I felt very similar to you when he first started going out - but then I oddly realised that I enjoyed the time I had without him, and I mentioned to him some of the things I'd like to start doing - I'm not big on going out, but, before kiddies I loved to sit and draw/paint, I loved long bubble baths and binge watching certain tv shows - not everybody's cup of tea but it's what I love!
So on the night he goes out, sometimes my MIL helps with the kids to give me time to myself, that's not always doable, so my partner will help me with the bedtime routine, he goes to his friends once that's done, and I either un myself a bath and pick something to watch, I get a takeaway sometimes too! Or i sit downstairs with my favourite music on quiet and I draw/paint 🥰
And on some weekends or if he has time off work, he sets an afternoon aside for me, I'm super chilled out about his hobbies, and sometimes they fully takeover and I don't get any time, so every now and then we have an afternoon for me, he gives me the option of what to do a whether it's an afternoon where he takes charge with parenting responsibilities and I have a bath, do my hair or even have a nap (sleepless nights kill me lol), or we go out as a family and do something I want to do, or he asks his MIL to help out, she takes the parenting responsibilities from us and we go somewhere I want to go!
The key is communicating with him your wishes and the things you want to do for you. Don't once you find a balance that works for you both, it's actually really nice!
Side note - there's even been times I've asked him if he wants to go out/make plans because I fancy a night of relaxing on my own once the bedtime routine is done with 🙈🙈