Denise Richards is mourning the loss of Patrick Muldoon in a touching tribute to her ex and former “Starship Troopers” costar.

The actress penned a lengthy letter on Instagram — accompanied by a reel of pictures and video of them together — Tuesday saying she was “deeply heartbroken” and “devastated” over his death.

As we previously reported … Patrick died after suffering a heart attack on Sunday morning.

In her post, Denise recalled how she and her former flame — who she affectionately called “Schweetbabe” — had known each other since they were in acting class together when she was 19 and he was 21.

She wrote that he’s been in her life for the past 36 years, to the point that she doesn’t know life without him. She wrote … “The friendship, love, support, respect & loyalty we had was unconditional and rare.”

DR reflected on Patrick’s “underrated” talent and his infectious energy, saying they’ve “been each other’s rocks.”

She continued to grieve him, adding … “You’ve had my back and protected me more than anyone. I don’t know what I’m going to do without you here. You’re my voice of reason & pillar of strength.”

Denise explained Patrick had been particularly helpful getting her through “pain & heartache this past year,” but now she needs him “more than ever, for this pain and heartache losing you.”

She mused how they’d talked about they’d “live forever,” and said she’ll be looking for signs from him … even now that he’s gone.

She concluded … “I need to know you’re still here with me. I love you so much & miss you dearly schweetbabe.”

Denise’s daughter, Lola Sheen, commented on the “beautiful” note, writing … “I am so so sorry. I love you so much, you guys are endgame forever.”

Remember … Lola had posted her own tribute to Patrick, saying that he was “supposed to be in our lives forever.”

Lola reflected that she had begged her mom to get back together with him … but ultimately decided “staying friends was better.”

Patrick was 57.