Feel like my boyfriend and I are drifting apart

Coming up to three years together and have lived together for one year. Everything was okay until this semester where his already difficult STEM course went from like 100-1000 real quick, and I tried my best to help out and make things easier for him (I do a Humanities course which is less intense). He gets back, does his uni work, eats, goes to bed. I try to chat to him on the occasions when he's not busy because I feel so lonely but he always seems distracted and stressed. I can't do anything extravagantly nice for him since we're both broke, except make sure my side of the bills are paid and keep the place tidy despite the fact we're both busy.

A month ago I confronted him about a sudden avoidant spell where he just wouldn't want to be near me for a few weeks, and he admitted that he'd been avoiding me because he just wanted time away. I asked him if his feelings towards me had changed and he said he wasn't sure anymore, leading to a night where I was distraught and thought we were going to break up. The next day, he told me he wasn't going to leave, and everything was fine again for a month but to be honest I haven't felt very secure since. I wonder if he's becoming depressed from this, he already has anxiety but he denies having depression.

Sorry if this is a mess I'm just saying what comes to mind. I'm sure there are things I can be doing to make things better but I really don't know, it just feels like we're coasting along, just existing in the same space. I know that's common with long-term relationships (not saying 3 years is a long time) and I could fully accept that change if it were to happen, but I can tell when the love is suddenly switched off and I'm just so scared.